OIL-FINDING 


OIL-FINDING 

AN  INTRODUCTION  TO  THE  GEOLOGICAL 
STUDY   OF   PETROLEUM 


BY 

E.  H.  CUNNINGHAM  CRAIG,  B.A,,  F.G.S. 


LATE  OF   H.M.    GEOLOGICAL   SURVEY 


WITH   AN   INTRODUCTION 

BY 

SIR   BOVERTON    REDWOOD,   BART. 

ADVISER    ON    PETROLEUM   TO   THE   ADMIRALTY,    HOME   OFFICE,   AND   INDIA   OFFICE 
CONSULTING   ADVISER   TO   THE  COLONIAL  OFFICE 


ILLUSTRATE? 


LONDON 

EDWARD   ARNOLD 
1912 

{All  rights  reserved} 


TO 
AKTHUR    SANKEY    KEID 

IN  GRATEFUL  MEMORY 
OF  HOW  MUCH  I  OWE  TO  HIM 


CONTENTS 


PAGE 

INTRODUCTION  BY  SIR  BOVERTON  REDWOOD,  BART.        .        .  vii 

PREFACE ix 

CHAPTER 

I.    THE  ORIGIN  OF  PETROLEUM    .        .        „        .        .  ~     .        .  l 

H.    PROCESSES  OF  FORMATION       . 25 

III.  THE  MIGRATION,  FILTRATION,  AND  SUBTERRANEAN  STORAGE 

OF  PETROLEUM .'      .  38 

IV.  LATERAL  VARIATION       .        .        .        .        .        .        ...  56 

V.    GEOLOGICAL  STRUCTURE 67 

VI.    INDICATIONS  OF  PETROLEUM    .......  88 

VII.    STRATIGRAPHY        ." 121 

VIII.    LOCATION  OF  WELLS       . 132 

IX.    (FOR  BEGINNERS)   FIELDWORK        .        .        ...        .  151 

X.    (FOR  BEGINNERS)   INDOOR-WORK     ......  176 

INDEX     .....  191 


254549 


INTRODUCTION 

BY 

SIR  BOVERTON   REDWOOD,   BART. 

MR.  CUNNINGHAM  CRAIG'S  "Oil-finding"  is  a  laudable  and 
successful  attempt  to  deal  with  a  subject  which  has  hitherto 
received  far  too  little  attention,  and  those  who  have  within 
recent  years  had  occasion  to  deplore  the  waste  of  money  which 
has  resulted  from  the  publication  of  injudicious  reports  on 
lands  presumed  to  be  oil-bearing,  and  from  the  unscientific 
manner  in  which  drilling  operations  have  been  conducted,  will 
regret  that  this  book  was  not  published  long  ago. 

The  author  points  out  that  the  foundation  of  the  successful 
petroleum  enterprise  must  be  laid  by  the  geologist  rather 
than  by  the  engineer,  and  he  states  that  the  present  work  has 
been  written  for  geologists,  and  especially  for  young  geologists ; 
but  it  may  be  added  that,  to  a  large  extent,  the  views  expressed 
are  couched  in  language  so  free  from  technicalities  that  the 
work  may  be  studied  with  profit  by  a  far  wider  circle,  including, 
in  fact,  all  those  who  are  interested  in  the  petroleum  industry, 
either  in  an  administrative  capacity,  or  as  investors. 

With  the  aid  of  this  book,  and  the  exercise  of  common 
sense,  those  who  contemplate  investment  in  petroleum  under- 
takings may  place  themselves  in  a  position  to  form  an  in- 
dependent opinion  as  to  whether  the  technical  data  given  in 
a  prospectus  are  adequate,  and  are  such  as  to  justify  the  appeal 
for  subscriptions.  Similarly,  the  shareholders  in  petroleum 
projects  which  are  of  an  exploratory  nature,  or  in  which  the 
work  of  exploitation  is  passing  through  the  earlier  stages,  may 
learn  to  interpret  reports  of  progress  which  at  present  they 
find  unintelligible.  Not  only  would  this  enlightened  judgment 
be  of  inestimable  value  to  those  who  exercise  it,  but  it  would 
incidentally  provide  the  most  effective  remedy  for  an  evil  to 


viii  INTRODUCTION 

which  the  author  alludes,  viz.  that  of  the  <;  popular  safeguarded 
report/'  which  in  lieu  of  being  a  record  of  facts  and  legitimate 
deductions,  seeks  to  create  a  highly  favourable  impression  by 
means  of  a  few  well-rounded  sentences,  in  which  the  weak 
points  of  the  case  are  ignored  and  superlatives  are  prominent. 
It  is  obvious  that  the  possession  by  those  to  whom  such  reports 
are  intended  to  appeal  of  such  knowledge  as  Mr.  Cunningham 
Craig's  book  imparts,  would  very  soon  result  in  rendering  this 
procedure  worse  than  useless,  and  in  this  connexion  it  should 
be  borne  in  mind  that  the  expert  is  not  alone  to  blame,  for, 
as  the  author  points  out,  it  not  infrequently  happens  that 
his  views  are  quoted  in  a  form  in  which  they  do  not  correctly 
convey  what  he  stated. 

The  earlier  portion  of  the  work,  as  the  author  himself 
admits,  deals  with  many  theoretical  questions  of  a  controversial 
character,  and  it  is  not  to  be  expected,  nor  does  he  himself 
anticipate,  that  his  strongly  expressed  opinions  will  in  all  cases 
be  accepted ;  but  this  does  not  detract  from  the  value  of  the 
work,  and  in  fact  it  may  be  said  to  enhance  it  if,  in  accordance 
with  the  avowed  intention  of  the  author,  the  further  study 
of  these  questions  is  thereby  stimulated. 

Mr.  Cunningham  Craig  in  his  concluding  remarks  modestly 
disclaims  originality  for  the  last  two  chapters  of  this  work, 
and  further  indicates  elsewhere  that  these  chapters  are  in- 
tended for  beginners,  but  the  dominant  feature  of  the  whole 
work  may  be  said  to  be  the  boldness  and  originality  of 
treatment  of  the  subject,  and  there  is  much  in  the  advice 
which  he  gives  to  beginners  which  may  be  studied  with 
advantage  by  those  who  have  had  lengthy  experience, 

B.  E. 


AUTHOR'S   PREFACE 

THIS  little  book  does  not  pretend  to  be  a  treatise  upon 
Petroleum,  nor  even  to  exhaust  the  particular  circumscribed 
branch  of  the  subject  with  which  it  deals. 

Petroleum  and  the  search  for  petroleum  have  recently 
bulked  largely  in  the  public  eye.  Discoveries  all  over  the 
world  have  proved  the  wide-spread  occurrence  of  mineral  oils ; 
the  demand  for  them  has  stimulated,  and  to  some  extent 
created,  active  search  for  and  development  of  petroliferous 
areas,  and  it  has  become  increasingly  evident,  not  only  to  the 
scientific,  but  also  to  the  commercial  world,  that  it  is  to  the 
geologist  rather  than  to  the  engineer  that  one  must  look  in 
the  first  instance  if  successful  results  are  to  be  achieved. 

Much  has  been  written  in  late  years  upon  the  subject  of 
petroleum,  but  very  little  that  is  of  service  to  the  practical 
field-geologist.  Speculative  and  more  or  less  theoretical  work 
by  indoor  students  of  the  subject  is  available  in  abundance, 
while  the  practical  work  of  oilfields  has  been  in  most  cases  con- 
ducted by  what  have  been  known  by  a  curious  distinction  (too 
frequently  heard  even  now-a-days),  as  "  practical "  as  contrasted 
with,  or  even  opposed  to,  "  scientific  "  men ;  consequently  many 
facts  of  vital  importance,  and  many  generalizations  reached  by 
long  experience  never  see  the  light  in  publications  accessible 
to  the  scientific  student  of  the  subject.  It  is  hardly  too  much 
to  say  that  so  much  nonsense  has  been  written  and  published 
about  oil,  or  particular  oilfields,  that  many  vague  but  essentially 
erroneous  ideas  are  current,  if  not  actually  accepted.  In  many 
otherwise  excellent  works  on  the  subject  geology  is  dismissed 
in  a  few  carefully  guarded  and  colourless  paragraphs,  or  at 
the  best  in  a  chapter  or  two,  while  such  data  as  statistics  are 
given  a  prominence  which  is  more  acceptable  to  the  commercial 
than  to  the  scientific  world.  Similarly,  geographical  data  are 
set  forth  at  length,  and  geological  maps  are  rarities. 


x  AUTHOR'S    PREFACE 

It  is  to  fill  up  a  few  of  the  blanks  left  by  previous  authors 
that  this  book  has  been  written ;  it  is  for  geologists,  and  more 
especially  for  young  geologists,  in  the  hope  that  it  may  prove 
of  value  to  those  who  may  have  to  undertake  exploration  work 
for  a  mineral  which  is  not  naturally  familiar  to  the  stay-at-home 
Briton. 

The  author  is  quite  aware  that  his  knowledge  of  petro- 
leum is  limited,  but  in  the  few  not  unimportant  fields  that 
have  come  within  his  ken  a  thorough  and  intimate  know- 
ledge has  been  obtained,  and  he  has  been  able  to  study  the 
subject  in  these  fields  in  a  manner  for  which  neither  the 
<f practical"  prospector,  nor  field-manager,  nor  the  peripatetic 
expert,  has  time.  Also  in  these  few  fields  he  has  had  an 
opportunity  not  only  of  observing  almost  all  the  different 
environments  in  which  petroleum  occurs,  and  many  different 
grades  and  classes  of  oils,  but  also  of  having  brought  to  his 
notice  nearly  every  kind  of  mistake  that  prospectors  or  experts 
with  a  merely  nodding  acquaintance  with  geology  can  make 
in  attempting  the  development  of  a  new  field. 

Theoretical  matters  will  perforce  have  to  be  treated  of, 
or  touched  on,  here  and  there,  but  for  the  most  part  the 
notes  that  follow  are  a  record  of  facts,  slowly  and  often 
laboriously  collected  during  detailed  geological  field-mapping, 
with  the  conclusions  to  which  these  facts  inevitably  point. 
If  the  author  has  taken  a  somewhat  uncompromising  stand- 
point with  regard  to  some  of  the  main  theoretical  questions 
still  under  controversy,  it  is  largely  with  the  intention  of 
stimulating  discussion  upon  them,  and  the  search  for  and 
bringing  forward  of  new  and  incontrovertible  evidence. 

It  is  assumed  that  the  reader  is  conversant  with  the  simple 
technical  terms  employed  in  field-geology. 

I  am  indebted  to  Messrs.  Eobert  Lunn  and  J.  Ness,  of 
H.M.  Geological  Survey,  for  help  in  the  preparation  of  the 
illustrations,  and  my  thanks  are  due  to  Messrs.  C.  S.  Rogers, 
M.  Crouliansky,  G.  B.  Reynolds,  S.  Lister  James,  and  Professor 
J.  Cadman,  who  have  kindly  allowed  me  to  publish  photographs 
of  geological  interest  which  they  have  taken. 

E.  H.  C.  C. 
April,  1912. 


LIST   OF   PLATES 

PLATE 

I.    THE  TWINGON  OIL  RESERVE,  YENANG YOUNG   FIELD,  BURMA 

Frontispiece 

FACING  PAGE 

II.      MOLLUSCA     FROM     THE     PEGU     SERIES     OF     BURMA,     SHOWING 

THE  DEATH-MARK 10 

III.  (i.)   SMOULDERING  ODTCROP  NEAR  LA  BREA  .        .        .        .18 
(ii.)  MUD-VOLCANO  IN  ERUPTION,  TRINIDAD    .        .  18 

IV.  THE  MARMATAIN  FIELD,  PERSIA 34 

V.    THE  \VHITE  OIL  SPRINGS  AT  KALA  DERIBID,  PERSIA   .        .      42 

VI.     THE  WHITE  OIL  SPRINGS  AT  KALA  DERIBID,  PERSIA   .        .  43 

VII.     THE  PERSIAN  OILFIELDS,  NEAR  KALA  DERIBID     ...  69 

VIII.     AN  ANTICLINE  IN  THE  MAIDAN-I-NAPHTUN  FIELD,  PERSIA    .  71 

IX.    (i.)   THE  LARGEST  MUD-VOLCANO   AT  MINBU,  UPPER  BURMA  102 

(ii.)  GROUP  OF  MUD-VOLCANOES  AT  MINBU     ....  102 

X.    (i.)  BUBBLE    BURSTING   IN   THE   CRATER   OF   THE    LARGEST 

MUD-VOLCANO   AT    MlNBU,    UPPER   BURMA    .  .  .       106 

(ii.)  PART    OF    THE    CRATER    OF    A    LARGE    MUD- VOLCANO 

("  CHEMIN  DE  DIABLE  "),,  TRINIDAD    .        .        .        .106 

XI.    A  CORNER  OF  THE  TWINGON  OIL  RESERVE,  BURMA      .        .     124 

XII.    THE  PUMP  STATION,  NYAUNGHLA,  UPPER  BURMA  .        .         .     132 

XIII.     A  CANADIAN  STEEL  DERRICK  AT  MINBU,  UPPER  BURMA      .     148 


OIL-FINDING 

CHAPTER  I 
THE   ORIGIN   OF   PETEOLEUM 

IT  has  been  the  custom  in  treatises  upon  petroleum,  the  one 
bright  exception  being  the  work  of  Engler  and  Hofer,  to  regard 
the  origin  of  the  mineral  as  an  interesting  academic  but 
unimportant  question,  which  is  fully  dealt  with  when  the 
various  theories  have  been  stated,  the  least  popular  summarily 
dismissed,  and  a  few  pages  of  carefully  guarded  general  state- 
ments written  round  about  without  ever  touching  the  root  of 
the  matter. 

That  this  unsatisfactory  state  of  things  has  arisen  from 
lack  of  definite  information  on  definite  points  is  the  misfortune 
rather  than  the  fault  of  the  authors,  but  all  geologists  will 
agree  that  to  leave  a  question  of  such  vital  importance 
unsettled  is  to  blindfold  the  student  or  prospector  at  the  very 
start.  How,  if  one  does  not  know,  or  cannot  prove,  from  what 
material  and  under  what  conditions  petroleum  is  formed,  can 
one  tell  whore  to  look  for  it  or  ever  be  confident  as  to  its 
presence  beneath  the  surface  ? 

This  question  of  origin,  in  fact,  absorbs  and  includes  nearly 
every  other  question  as  to  the  occurrence,  distribution,  and 
winning  of  oil. 

The  theories  that  have  been  brought  forward  from  time  to 
time  to  account  for  the  origin  of  petroleum  and  its  congeners 
divide  themselves  naturally  into  two  classes,  which  again  may 
be  subdivided  as  follows  : — 

A    T  .    ~  .  .  1.  Hypogene  Causes. 

A.  Inorganic  Origin     <   0    T/,r     •     A   .• 

2.  Volcanic  Action. 

.    ~  .  .         (  1.  From  Animal  Matter 

B.  Organic  Origin       •    n  •  __         .,    __     • 

2.  From  Vegetable  Matter. 


2  OIL-FINDING 

A.  Theories  of  the  inorganic  origin  of  petroleum  are 
essentially  the  ideas  of  chemists  and  indoor-students  of  the 
subject.  They  are  founded  on  assumptions  and  built  up  by 
theoretical  considerations,  none  of  which  have  been  tested  by 
application  to  actual  facts  and  conditions  as  observed  in 
nature. 

(1)  The   most  ingenious  of  the  Tiypogene  theories  suggests 
the  origin  of  petroleum  by  the  condensing  and  isomerization 
of    hydrocarbons  formed   by  the  action   of  water  upon   sup- 
posititious masses  of  metallic  carbides  deep  within  the  crust 
of  the  earth.     Such  vague    hypotheses  are  almost  invariably 
rejected  nowadays,  and  it  is  needless  to  enter  upon  a  detailed 
examination    of    the    various   arguments  pro   and  con.      The 
conditions  under  which  petroleum  is  found  in  nature  furnish 
sufficient  grounds  for  the  dismissal  of  any  theory  involving  a 
deep-seated  origin. 

(2)  Volcanic  action  has  occasionally  been  suggested  to  be 
responsible    in   some    ill-defined   way   for   the   occurrence   of 
petroleum.     At   first   sight  there  appears  to  be  some  direct 
evidence  favourable  to  this  idea.     For  instance,  oilfields  are 
found  in  many  parts  of  the  world  at  no  great  distance  from, 
and   even  running  parallel   with,   lines   of   volcanic   activity. 
Japan   and   Sakhalin,   Mexico,  Burma,  and  the  West  Indies 
are  cases  in  point.     Again,  mud-volcanoes  of  solfataric  type, 
an   evidence    of   undoubted    volcanic    action    usually   in   the 
obsolescent  stage,  have  been  confused  with  mud-volcanoes  due 
to  the  discharge  of  gas  from  underlying  oil-rocks.     These  are 
two  entirely  different  phenomena,  but  if  no  distinction  be  made 
between  them  it  might  be  erroneously  claimed  that  there  is 
evidence  of  volcanic  action  in  very  many  oilfields.     When  the 
question  is  examined  in  detail,  it  is  seen  that  both  the  lines 
of  volcanic  activity  and  the  structures  which  are  conducive  to 
the  formation  and  storage  of  petroleum  are  merely  separate 
and   independent  effects   of  the  same  cause.     Volcanic  lines 
are  developed  near  to   or  along  the   margins   of  continental 
masses,    or,    more    correctly,   between    continents    and   deep 
oceanic  basins;   that  is  to  say,  in  belts  where  active  earth- 
movement  is  taking  place.     Many  oilfields  also  lie  along  belts 
where  active  earth-movements  have  been  experienced,  but  there 
is  no  evidence  to  suggest  that  the  vulcanicity  and  the  formation 
of   petroleum  r  are    essentially,  connected   in   any   way.      The 


THE   ORIGIN   OF   PETROLEUM  3 

distribution  of  land  and  water  may  be  vastly  different  now 
from  what  it  was  when  active  vulcanicity  obtained ;  and  it 
can  frequently  be  proved  that  the  oil  was  formed  before 
vulcanicity  commenced,  and  may  have  remained  after  all  such 
action  has  ceased.  Interesting  evidence  of  this  nature  is  to 
be  obtained  in  Burma  and  Barbados. 

To  go  into  the  matter  more  closely,  it  is  difficult  for  a 
geologist  to  realize  exactly  what  those  authors  who  have 
promulgated  the  idea  of  a  volcanic  origin  of  petroleum  really 
mean  by  "  volcanic  action."  As  evidence  they  bring  forward 
the  well-known  cases  of  distillation  caused  by  the  intrusion 
of  igneous  rocks  through  such  strata  as  the  coal  measures  or 
the  Scottish  oil-shales — phenomena  which  are  not  necessarily 
connected  with  true  volcanic  action.  That  such  local  dis- 
tillation has  frequently  taken  place  is  proved  by  abundant 
evidence,  and  the  igneous  rock  may  be  found  to  contain  in 
small  cavities  soft  or  elastic  bituminous  compounds.  In  Fife 
and  in  the  Karroo,  South  Africa,  such  evidence  may  be 
obtained. 

These  effects,  however,  are  purely  local,  and  no  instance  of 
such  action  on  a  large  scale  has  been  brought  forward;  the 
limits  of  the  distilling  action  are  usually  well-defined.  Even 
were  such  evidence  on  a  large  scale  available,  the  occurrence 
of  the  bituminous  compounds  only  demonstrates  the  presence 
of  organic  matters  contained  within  the  sedimentary  strata  so 
affected,  and  the  igneous  action  cannot  be  considered  the  origin, 
but  merely  the  process  which  has  happened  to  call  attention  to 
the  potentially  bituminous  nature  of  the  strata. 

That  volcanoes  themselves  should  produce  petroleum  has 
not  been  suggested,  since  in  spite  of  the  minute  care  with 
which  volcanoes  have  been  studied,  no  observer  has  obtained 
evidence  favourable  to  such  an  hypothesis. 

In  no  large  and  productive  oilfields  have  igneous  rocks, 
either  intrusive  or  volcanic,  been  encountered  to  any  con- 
siderable extent,  and  to  use  the  term  "  volcanic  action  "  in  a 
vague  sense  to  account  for  phenomena  which  are  not  associated 
with  any  such  action  is  merely  to  beg  the  question,  and  at  the 
same  time  to  disregard  a  mass  of  relevant  evidence  which  has 
been  gradually  accumulated  ever  since  petroleum  first  became 
of  commercial  importance. 

B.   Organic    Origin. — In   considering   the   theories   of    the 


4  OIL-FINDING 

formation  of  petroleum  from  organic  matter  it  is  necessary 
to  examine  a  vast  mass  of  evidence,  chemical,  geological,  and 
experimental.  The  views  of  many  experts  are  still  undecided, 
and  the  relative  importance  of  animal  or  vegetable  matter  as 
the  material  from  which  petroleum  and  petroleum  compounds 
can  be  formed  is  a  question  that  is  handled  very  gingerly  in 
recent  publications.  Yet,  as  stated  above — and  it  cannot  be 
stated  too  often  and  too  strongly — unless  we  can  make  up  our 
minds  upon  this  question,  the  search  for  new  oilfields  must 
necessarily  become  to  some  extent  a  groping  in  the  dark. 
Before  asking  himself  if  there  is  oil  to  be  found  in  any  district 
or  locality,  the  geologist  must  consider  why  there  should,  or 
should  not,  be  oil ;  how  it  could  have  reached  such  an  environ- 
ment, and  whether  it  can  be  relied  upon  to  be  present,  if 
drilled  for.  To  enter  into  such  enquiries  without  knowing 
from  what  material  the  oil  has  been,  formed,  is  to  adventure 
upon  a  search  with  one  eye  bandaged. 

The  question  is  not  merely  of  academic  but  of  great 
practical  importance,  and  it  is  in  the  author's  experience  that 
great  sums  of  money  have  been  fruitlessly  thrown  away  by 
petroleum  companies  solely  because  those  responsible  for  the 
selection  of  possible  new  fields  to  be  tested  had  not  mastered 
this  first  essential  question  of  the  origin  of  petroleum. 

B  (1)  Animal  Origin. — The  theory  that  petroleum  is  formed 
by  the  decomposition  or  destructive  distillation  of  animal  matter 
entombed  in  the  strata  has  many  adherents  at  the  present  day. 
It  has  arisen  largely  from  the  wish  to  find  a  marine  origin  that 
will  be  acceptable  to  scientists.  As  oil  occurs  in  sedimentary 
strata,  and  most  sedimentary  strata  are  to  some  extent  at  least 
marine  in  origin,  there  was  a  natural  tendency  to  seek  for  some 
mode  of  origin  for  petroleum  compatible  with  the  manner  of 
accumulation  of  ordinary  sedimentary  rocks. 

The  evidence  upon  which  this  theory  rests  is  largely 
chemical,  many  chemists  having  conducted  researches  with 
the  object  of  ascertaining  whether  oils  with  the  character- 
istics of  natural  petroleum  can  be  formed  from  animal 
matter. 

Let  it  be  granted  at  once  that  under  conditions  easily 
reproducible  in  a  chemical  laboratory  animal  matter  of  almost 
every  kind  can  be  decomposed  and  separated  out  into  various 
classes  of  compounds,  some  of  which  can,  when  properly 


THE   ORIGIN   OF   PETROLEUM  5 

treated,  be  converted  into  mixtures  of  oils  closely  resembling 
petroleum  as  found  in  nature. 

The  chemical  processes  can  be  expressed  roughly  and 
generally  as,  first,  the  elimination  of  nitrogen  and  nitrogen 
compounds,  and  then  a  destructive  distillation  of  the  fats 
to  form  mixtures  of  hydrocarbons.  With  the  actual  processes 
employed  it  is  not  necessary  to  deal  in  detail ;  the  reader  is 
referred  to  the  work  of  Engler  and  Hofer  and  of  others  who 
have  made  the  possibility  of  the  extraction  of  oils  from  animal 
matter  abundantly  clear. 

This  possibility  being  accepted,  many  authors,  pointing  to 
the  indubitable  evidence  of  the  former  existence  of  living 
organisms  among  the  strata  in  which  petroleum  is  now  found, 
seem  to  consider  that  further  proof  is  unnecessary.  Each 
author  has  probably  his  favourite  class  or  order  of  organism 
which  he  would  make  responsible  for  the  raw  material  in  each 
particular  oilfield  of  which  he  has  special  knowledge.  Thus 
at  one  time  or  another  almost  every  class  of  organism,  from  the 
fish  of  Mr.  Winda,  the  Eussian  geologist,  to  the  (dia-tqmi;  and 
foraminifera  of  the  United  States  Geological  Survey  (California, 
Texas  and  Louisiana  Oilfields)  has  had  special  attention  drawn 
to  it  in  this  connection. 

Sometimes  the  chemical  theorists  carry  their  speculations 
even  further,  and  suggest  that  the  characteristics  of  the  oils 
formed,  e.g.  whether  of  paraffin  or  asphaltic  base,  may  be 
determined  by  the  nature  of  the  raw  material. 

Of  the  geological  evidence,  however,  little  that  will  bear 
careful  scrutiny  has  been  adduced  to  support  the  animal-origin 
theory.  A  statement  such  as  the  following,  "this  series  is 
oil-bearing,  and  at  intervals  throughout  it  the  hard  parts  of 
animal  organisms  are  found,"  seems  to  be  regarded  by  many  as 
sufficient  evidence  upon  which  to  base  a  generalization  of  such 
enormous  importance. 

Again,  the  occurrence  of  oil  in  limestones  is  often  brought 
forward  as  a  clinching  argument,  even  by  authors  who,  perhaps 
in  the  next  chapter,  deal  with  the  migration  of  petroleum 
through  vast  thicknesses  of  strata  and  its  appearance  naturally 
in  the  most  porous  rock  available.  "  Here,"  one  will  say,  "  is 
a  coral  limestone  formed  chiefly  of  the  debris  of  coral  and  other 
organisms  often  in  the  position  of  growth,  and  it  is  impregnated 
with  petroleum,"  leaving  it  to  be  inferred  that  the  oil  has  been 

\ 


6  OIL-FINDING 

formed  from  the  soft  parts  of  the  coral  polyps,  and  oblivious 
of  the  fact  that  a  similar  argument  might  be  made  to  apply  to 
a  recent  beach,  full  of  shell  fragments  and  similarly  impreg- 
nated, such  as  may  be  seen  in  many  localities  in  the  Island 
of  Trinidad. 

The  geologist,  however,  demands  more  detailed  and  definite 
evidence;  it  is  not  enough  for  him  to  know  where  the  oil  is 
found,  he  must  assure  himself  on  many  points,  such  as  the 
lateral  and  vertical  distribution  of  the  petroleum  in  a  geological 
series,  the  conditions  under  which  the  series  has  been  deposited, 
the  manner  in  which  sufficient  raw  material  to  form  the  oil  has 
been  accumulated,  and  the  process  by  which  the  oil  has  been 
concentrated  and  brought  to  its  present  position.  When  such 
questions  are  gone  into  carefully,  one  possibility  after  another 
is  disposed  of,  and  by  a  process  of  elimination  an  inevitable 
conclusion  is  finally  reached. 

In  such  enquiries  the  golden  rule  is  never  to  postulate  or 
suggest  any  condition  or  any  mode  of  deposition  or  accumulation 
which  cannot  be  shown,  or  proved,  to  be  actually  in  operation  at 
the  present  day.  It  is  by  the  study  of  the  present  that  the 
secrets  of  the  past  are  revealed. 

In  justice  to  the  chemical  theorists  it  must  be  admitted 
that  they  have  occasionally  attempted  to  meet  the  objections  of 
geologists  by  reference  to  actual  facts.  Samples  of  sludge  or 
slimy  mud  containing  organic  matter  more  or  less  decomposed 
have  been  taken  from  harbours,  estuaries,  or  mud- flats,  analysed 
and  distilled,  and  petroleum-like  compounds  in  minute  quanti- 
ties, it  is  true,  separated  out.  The  fallacy  lies  in  the  assumption 
that  these  samples  from  the  upper  layers  of  the  sludge  are 
typical  in  chemical  composition  of  the  mass  of  slowly  accumu- 
lating material  beneath.  The  upper  layers  teem  with  animal 
life,  no  doubt,  but  there  is  a  rapid  change  downwards.  When 
a  dredger  is  working  in  the  sludge  of  a  harbour  or  estuary,  it 
will  be  observed  by  anyone  who  makes  a  study  of  the  material 
removed  that  the  lower  layers  differ  very  considerably  in  colour 
from  the  upper  layers,  and  that  at  a  depth  of  two  or  three  feet 
almost  all  trace  of  organic  matter,  with  the  exception  of  the 
hard  parts  of  mollusca,  has  disappeared.  The  change  of  colour 
is  almost  entirely  due  to  the  reduction  of  iron  compounds, 
ferrous  salts  replacing  ferric,  and  this  process  is  effected 
principally  by  the  decomposition  of  organic  matter.  The  author 


THE    ORIGIN    OF   PETROLEUM  7 

had  occasion  at  one  time  to  note  day  by  day  samples  of  the 
slowly  accumulating  fine  sludge  of  Port  of  Spain  Harbour, 
Trinidad.  These  samples  were  taken  on  the  Government 
dredger,  and  a  selection  of  them  was  analysed  by  Professor 
Carmody,  Government  Analyst  of  Trinidad.  The  environment 
is  an  ideal  one  for  the  accumulation  of  animal  matter  and  its 
entombment  in  impervious  argillaceous  sediment.  But  in  the 
specimens  analysed  the  percentage  of  organic  matter  was 
infinitesimal,  though  the  remains  of  the  hard  parts  of  mollusca 
were  by  no  means  uncommon.  Such  sludges  will  become  in 
time  blue  clays,  precisely  similar  to  those  which  are  so  frequent 
among  the  Tertiary  strata  of  Trinidad,  and  which,  though  they 
often  contain  rich  molluscan  faunas,  are  almost  entirely  free 
from  organic  matter. 

It  is  doubtful,  indeed,  if  it  is  ever  possible  for  the  soft  parts 
of  animal  organisms  to  be  entombed  to  any  considerable  extent 
among  accumulating  sediments.  In  seas  and.  estuaries  the 
waters  and  the  upper  layers  of  whatever  sediment  is  being 
formed  teem  with  life,  but  as  each  organism  dies  it  is  eaten  or 
decomposed — in  most  cases  it  is  certainly  eaten  alive.  Its  soft 
parts  become  absorbed  into  the  bodies  of  living  organisms, 
only  its  hard  parts  (and  often  not  very  much  of  them)  go  to 
swell  the  deposit  of  sedimentary  material.  Thus  equilibrium 
is  maintained ;  the  mass  of  organic  matter  does  not  go  on 
indefinitely  increasing,  but  remains  a  practically  constant 
quantity ;  the  inorganic  matter  is  continually  being  extracted 
by  the  living  organisms  from  the  water  and  the  sediment 
brought  into  it  by  rivers  and  by  denudation  of  the  coast-line, 
and  this  inorganic  matter,  after  a  longer  or  shorter  period  in 
which  it  is  part  of  a  living  organism,  is  being  passed  on  to  take 
its  part  in  the  formation  of  future  strata. 

Thus  the  first  great  difficulty  that  upholders  of  the  animal- 
origin  theory  have  to  face  is  that  of  proving  that  animal  matter 
can  be  entombed  in  sufficient  quantity  to  account  for  the  vast 
stores  of  petroleum  contained  in  sedimentary  strata.  It  is 
possible,  of  course,  under  special  local  conditions,  to  preserve 
and  entomb  the  soft  parts  of  animals,  but  throughout  the 
geological  record  instances  of  such  preservation  are  very  few 
and  far  too  insignificant  to  serve  as  evidence  against  the  known 
facts  as  to  the  almost  universal  destruction  or  decomposition 
that  overtakes  each  organism  sooner  or  later. 


8  OIL-FINDING 

To  point  to  highly  fossiliferous  strata  as  proof  that  auimal 
matter  has  been  entombed  in  large  quantities  is  to  disregard 
facts  for  the  sake  of  an  attractive  theory.  The  hard  parts  of 
diatoms  and  foraminifera  cannot  sink  and  become  involved  in 
a  sedimentary  deposit  till  the  animal  matter  has  been  destroyed, 
and  similarly  nearly  every  fossil  that  is  preserved  in  strata  can 
be  proved  to  have  lost  its  soft  parts  before  becoming  incorporated 
in  a  bed  that  is  being  formed. 

An  interesting  illustration  of  this  process  of  nature  may  be 
studied  in  almost  any  fine  argillaceous  rock  rich  in  fossil  evi- 
dence; a  clay  for  instance  that  has  accumulated  slowly  and 
that  now  contains  perfect  specimens  of  the  hard  parts  of 
rnollusca,  such  as  lamellibranchs  with  the  two  valves  joined 
and  closed  or  gasteropods.  In  such  a  case,  if  anywhere,  it 
might  be  expected  that  entombment  of  animal  matter  might 
have  taken  place.  But  what  do  we  find?  Almost  every 
perfect  specimen  bears  the  "death-mark"  (Plate  II),  the  small 
round  hole  drilled  by  the  predatory  gasteropod,  which  has 
fastened  upon  its  victim,  pierced  its  outer  armour,  and  devoured 
its  fleshy  part.  Fossiliferous  clays  in  the  Pegu  Series  of  Burma, 
a  series  in  -which  oil-bearing  strata  occur  at  intervals  through- 
out a  thickness  of  4000  feet  in  some  localities,  afford  very 
abundant  evidence  of  this  nature. 

The  most  fossiliferous  beds,  however,  are  almost  invariably 
littoral' deposits  in  which  there  is  a  mixture  of  forms  from  deep 
and  shallow  water.  Whether  whole  or  fragmentary,  many  of 
the  fossils  show  the  death-mark,  while  the  fragmentary  nature 
of  most  specimens  proves  that  they  have  been  washed  about 
the  shore  with  every  wave  and  tide  long  after  they  have  lost 
all  traces  of  their  soft  parts.  The  abundant  evidence  of  Crustacea 
and  fishes,  the  scavengers  of  the  sea,  included  in  such  beds 
serves  to  remind  us  that  organic  material  is  not  wasted,  is 
not  rejected  to  pass  with  inorganic  matter  into  the  sediment, 
but  is,  so  to  speak,  continually  kept  in  circulation.  "Eat 
and  be  eaten"  is  a  law  of  nature,  and  from  these  little  life 
tragedies  of  the  mollusc  we  may  realize  the  difficulty  in  the 
way  of  the  accumulation  of  sufficient  animal  matter,  a 
difficulty  which  the  animal-origin  theorists  gloss  over  so  light- 
heartedly. 

Quite  apart  from  this  initial  impossibility  of  proving  a 
sufficient  supply  of  raw  material  from  which  petroleum  might 


THE   ORIGIN    OF    PETROLEUM  9 

be  formed,  there  is  also  much  chemical  evidence  against  the 
animal-origin  theory. 

It  is  only  from  the  fatty  parts  of  animal  organisms  that  the 
petroleum  could  be  formed,  so  it  is  only  a  portion,  and  often 
a  very  small  portion,  of  the  soft  parts  that  can  be  utilised.  The 
elimination  of  nitrogenous  compounds  and  at  the  same  time 
the  preservation  of  the  fats  must  be  presupposed,  and  such 
an  assumption  may  be  said  to  beg  the  whole  question.  The 
theory  is  that  the  animal  matter  decomposes  in  such  a  manner 
that,  before  it  is  entombed,  practically  all  nitrogenous  matter 
has  been  removed  (since  only  the  merest  traces  of  nitrogen 
compounds  have  ever  been  found  in  natural  petroleum),  and 
the  preservative  action  of  salt  water  has  even  been  adduced 
to  make  such  a  retarded  decomposition  appear  less  improbable. 
But  can  we  find  any  evidence  of  such  a  selective  decomposition 
in  nature  ?  Are  fats  preserved,  even  in  sea- water,  while  flesh 
is  decomposed  and  dissipated  as  gases  ?  Let  any  one  who  has 
studied  the  formation  of  guano,  or  who  has  been  unfortunate 
enough  to  have  the  processes  in  the  decomposition  of  a  dead 
whale  forced  upon  his  senses,  answer. 

A  very  special  and  peculiar  form  of  decomposition  must 
be  postulated,  and  furthermore,  one  that  does  not  eliminate 
the  sulphur  content,  since  sulphur  compounds  are  in  many 
cases  present  in  petroleum,  sometimes  in  large  quantity.  The 
high  pressures  necessary  to  favojiir  the  formation  of  paraffin 
hydro-carbons  from  fats  are  inconsistent  with  conditions  that 
will  allow  the  escape  of  nitrogen  in  gaseous  compounds,  and 
as  it  is  neither  expedient  nor  justifiable  to  assume  the  existence 
of  conditions  of  which,  we  have  no  actual  evidence  in  nature, 
much  of  the  interesting  laboratory  evidence  can  only  be  regarded 
as  experimental  rather  than  explanatory. 

Another  difficulty  which  the  animal-origin  theorists  have 
to  encounter  is  the  disposal  of  the  phosphorous  contents  of  the 
animal  matter.  This,  of  course,  on  the  decomposition  of  the 
animal  organisms  naturally  takes  the  form  of  phosphates.  Now 
of  all  salts  formed  in  nature  the  phosphates,  whether  of  iron 
or  calcium,  or  double  and  compound  phosphates,  are  among  the 
most  difficult  to  dissolve  and  remove  in  solution.  Hence, 
phosphatic  beds  or  lines  of  phosphatic  nodules  may  be  expected 
near  or  among  those  beds  where  animal  organisms  have  been 
most  abundant.  The  phosphates  indeed  remain  chiefly  as,  or 


io  OIL-FINDING 

in,  the  hard  parts  of  the  organism  when  the  softer  parts  have 
been  decomposed  or  absorbed  into  the  economy  of  other  living 
organisms.  The  proportion  of  derived  phosphates  to  animal 
fats  is  very  high  in  nearly  all  marine  and  fresh- water  organisms. 
If,  then,  we  are  to  contend  that  the  petroleum  of  our  great  oilfields 
is  derived  from  animal  matter,  vast  stores  of  phosphate  must 
be  present  somewhere  in  the  vicinity  of  the  place  where  the  oil 
has  been  formed.  But  we  know  of  no  great  phosphatic  deposits 
associated  with  oil-rocks  or  within  the  confines  of  oilfields. 

This  objection  is  partially  met  by  the  suggestion  that  the 
oil  is  formed  in  very  minute  quantities  throughout  very  wide- 
spread deposits  of  enormous  thickness,  and  has  been  gradually 
concentrated,  migrating  drop  by  drop  to  where  it  is  now  found ; 
and  as  phosphates  occur  in  small  quantities  in  practically  every 
rock,  sedimentary  or  igneous,  and  in  an  oilbearing  series  as 
elsewhere,  it  may  be  claimed  that  the  quantity  of  petroleum 
formed  in  any  given  area  is  not  out  of  all  proportion  to  the 
quantity  of  phosphates  in  that  area.  No  calculations  as  to  the 
proportion  of  phosphates  to  oil  have  been  put  before  the  scientific 
world  as  yet.  The  calculation,  however,  is  simple.  The  area 
from  which  an  oilfield  can  have  derived  its  petroleum  can  be 
demarcated  in  many  cases  with  considerable  accuracy,  the 
weight  of  oil  in  the  field  can  be  estimated,  as  has  often  been 
done,  and  the  average  phosphate  content  of  the  strata  can  be 
obtained  by  a  series  of  analyses. 

If  fish,  as  has  been  suggested,  are  the  source  of  origin  of  the 
petroleum,  the  proportion  of  phosphates  to  oil  must  be  very 
high,  and  even  if  lower  organisms  are  made  to  do  duty  as  the 
source  of  raw  material,  the  proportion  of  phosphate  to  animal 
fat  is  still  large.  It  will  be  found  that  such  an  enormous  mass 
of  phosphatic  material  would  have  to  be  postulated  as  existing 
in  the  strata,  that  its  presence  would  be  continually  demonstrated 
by  bed  after  bed  of  nodules  or  masses,  which  would  be  of  too 
great  commercial  value  to  have  been  overlooked. 

It  is  unnecessary  to  allude  to  more  of  the  practical 
difficulties  which  beset  the  animal  origin  theory  when  it  is 
tested  by  reference  to  geological  field  evidence.  To  sum  up,  those 
who  believe  in  an  animal  origin  for  petroleum  have  to  call  to 
their  aid  methods  of  accumulation  of  material  of  which  we  have 
no  evidence,  and  chemical  processes  easily  arranged  for  in  a 
laboratory,  but  of,  to  say  the  least,  very  doubtful  occurrence  in 


THE   ORIGIN   OF   PETROLEUM  11 

nature.  The  theory,  attractive  as  it  may  be  from  the  chemist's 
point  of  view,  fails  utterly  when  applied  practically:  the 
artificial  light  of  a  laboratory  is  more  favourable  to  its  develop- 
ment than  the  cold  light  of  facts  gathered  in  the  field. 

B  (2)  Vegetable  Origin. — There  remains  the  theory  of  forma- 
tion from  vegetable  matter.  In  one  form  or  another  this  theory 
has  been  in  existence  from  a  very  early  date  in  the  history  of 
oilfield  discovery  and  development,  and  it  is  held  now  by  many 
observers  who  have  had  to  study  oilfields,  but  I  am  not  aware 
of  its  having  been  stated  at  length  in  any  geological  treatise. 
Consequently  the  opponents  of  the  theory  not  uncommonly 
seem  to  labour  under  a  totally  wrong  impression  as  to  what  the 
vegetable-origin  theory  is,  and  it  has  even  been  stated  that 
distillation  from  coal  or  lignite  is  relied  upon  by  the  vegetable- 
origin  theorists  to  account  for  the  presence  of  petroleum,  an 
idea  to  which  no  practical  geologist  at  the  present  day  would 
attach  any  importance. 

Perhaps,  therefore,  it  will  be  as  well  to  state  briefly  what  is 
the  vegetable-origin  theory  as  understood  and  followed  by  the 
scientific  prospector  or  field  geologist,  before  proceeding  to  give 
a  review  of  the  mass  of  evidence  which  leads  inevitably  up 
to  it. 

Petroleum  is  formed  from  the  remains  of  terrestrial  vegetation, 
accumulated  in  clays,  sands,  or  actual  beds  (which  under  other 
conditions  would  develop  into  carbonaceous  shales,  sandstones, 
and  seams  of  coal  or  lignite),  ly  natural  processes  which  can  be 
not  only  reproduced  in  the  laboratory,  but  can  also  be  proved 
to  have  taken  place  in  the  past  and  are  taking  place  at  the 
present  day. 

In  weighing  this  theory,  as  in  the  case  of  the  animal-origin 
theory,  there  are  first  of  all  some  general  considerations  to  be 
dealt  with  to  ascertain  whether  there  be  any  inherent  improba- 
bility in  the  hypothesis  as  stated.  For  the  present  the  actual 
processes  by  which  petroleum  can  be  formed,  or  is  formed,  need 
not  be  considered. 

The  first  question  to  be  asked  is,  "  Can  sufficient  material 
be  accumulated  ? "  In  other  words,  is  it  possible  for  terrestrial 
vegetable  remains  to  be  distributed  throughout  sedimentary 
strata,  or  to  be  formed  into  beds  which  can  afterwards  be 
entombed  to  play  their  part  in  the  geological  record  ?  To  this 
question  every  coalfield  or  lignite-field,  every  carbonaceous 


12  OIL-FINDING 

shale  or  sandstone,  every  peatbog  or  buried  forest,  returns  an 
emphatic  affirmative. 

The  second  question  follows  naturally :  is  it  possible  from 
accumulations  of  vegetable  matter  to  form  bituminous  com- 
pounds or  hydrocarbons  by  natural  processes  ?  The  coalfields 
again  furnish  us  with  a  very  definite  answer.  In  nearly  every 
coalfield  of  importance,  and  especially  be  it  noted  where  deep 
coals  in  little  disturbed  strata  are  worked,  there  is  a  consider- 
able proportion  of  bitumen  in  one  or  more  seams.  Where  the 
coal  measures  are  most  disturbed  and  the  seams  crop  out,  the 
least  traces  of  bitumen  are  found.  Where  the  coals  are  most 
completely  sealed  up  by  impervious  shale  beds,  where  they  are 
buried  deeply  or  do  no.t  crop  out  at  the  surface,  the  proportions 
of  bitumen  and  gaseous  hydrocarbons  are,  ceteris  paribus, 
greater. 

To  this  it  may  be  objected  that  we  are  now  dealing  with 
oil  and  not  with  coal,  and  that  coal  and  oil  are  not,  as  a  rule, 
found  in  intimate  association.  This  point  will  be  dealt  with 
later ;  for  the  present  the  possibilities  alone  are  under  con- 
sideration. 

There  is,  then,  no  difficulty  about  the  accumulation  of 
sufficient  material,  and  material  of  a  suitable  kind. 

The  next  point  to  be  considered  is  under  what  conditions 
have  the  strata  associated  with  coal  or  lignite  seams  or  carbona- 
ceous shales  and  sandstones  been  deposited  and  consolidated, 
and  under  what  conditions  the  strata  associated  with  petroleum. 
This  is  a  matter  that  is  not  always  obvious  except  to  the 
practical  geologist.  It  used  to  be  objected  to  the  "growth-in- 
situ "  theory  of  coal  formation  that  the  fauna  of  the  coal 
measures  is  largely  marine,  but  when  stated  more  correctly 
this  objection  is  seen  to  have  little  weight.  It  should  be  read 
as  follows  :  In  the  coal  measures,  among  the  constant  alterna- 
tions of  thin  beds  of  shales,  sandstones,  coalseams,  etc.,  occur 
here  and  there  beds  containing  a  marine  (usually  a  littoral 
pelecypod)  fauna,  while  the  other  strata  are  mostly  unfossili- 
ferous  except  for  the  occurrence  of  terrestrial  organisms.  These 
marine  beds  are  frequently  mere  shell-banks  of  sandstone, 
calcareous  sandstone,  or  even  limestone  or  ironstone  (e.g. 
"  blackband  ") ;  they  are  thin,  and  liable  to  rapid  lateral  varia- 
tions, as  miners  of  the  blackband  seams  can  testify.  Speaking 
generally,  our  Carboniferous  coal  measures,  or,  indeed,  any 


THE   ORIGIN    OF   PETROLEUM  13 

coal  or  lignite  measures  in  any  part  of  the  world,  consist  of 
rapid  alternations  of  thin  beds  of  rapidly  accumulated  sediment, 
varied  with  occasional  bands  truly  marine  in  origin,  and 
horizons  denoting  terrestrial  conditions.  To  the  geologist  such 
evidence  spells  littoral,  estuarine,  or  deltaic  conditions  on  a 
large  scale. 

But  following  our  method  of  interpreting  past  conditions  by 
what  we  can  actually  see  in  operation  at  present,  let  us  consider 
in  what  parts  of  the  world  great  accumulations  of  vegetable 
matter  are  being  formed,  where  by  a  slight  change  of  level  marine 
conditions  may  be  brought  into  play  over  wide  areas,  and  so 
marine  strata  made  to  alternate  with  terrestrial.  The  only 
places  where  such  an  environment  obtains  on  a  large  scale  are 
in  the  deltas  of  great  rivers  such  as  the  Amazon,  Orinoco, 
Mississippi,  Ganges,  and  Brahmapootra,  Irrawaddy,  etc.,  and  in 
neighbouring  areas  where  the  same  phenomena  on  a  smaller 
scale  may  be  studied  with  a  greater  facility.  Within  the  mazes 
of  a  great  delta,  what  are  known  in  South  America  and  the 
West  Indies  as  "  lagoons,"  i.e.  swamp-forests  growing  at  sea- 
level,  separated  from  the  sea  only  by  a  fringe  of  sandbanks  or  a 
belt  of  mangrove  swamp,  cover  vast  areas.  In  these  swamps 
and  lagoons  the  accumulation  of  vegetable  matter  is  remarkably 
rapid,  while  in  times  of  flood  much  of  the  land  is  under  water, 
and  any  slight  movement  of  depression  would  cause  a  great 
advance  of  the  sea-margin  and  cause  marine  strata,  littoral 
sands,  and  fine  silts  and  clays  to  be  deposited  over  the  beds  of 
terrestrial  origin. 

In  the  much-written-about,  but  little  known,  Island  of 
Trinidad,  there  is  an  ideal  area  to  study  such  conditions  at  the 
present  day — mangrove  swamps,  forest  lagoons,  delta  formation, 
and  retreat  and  advance  of  the  sea  under  differential  earth- 
movements  of  very  recent  date.  At  sea  level  can  be  seen  in 
process  of  formation  terrestrial  deposits  separated  by  a  strip  of 
littoral  sands  from  truly  marine  silts  and  clays,  with  occasional 
coral  banks  in  reef  formation  which  will  eventually  form  lime- 
stones. Furthermore,  a  study  of  the  excellent  coast  sections  in 
that  island  makes  it  clear  that  precisely  the  same  conditions 
existed  throughout  the  Tertiary  period.  The  same  rapid  alter- 
nations in  sedimentary  types  are  seen,  lignite  seams  and  oyster 
beds,  littoral  sandstones  and  marine  silts,  thin  calcareous  bands 
and  ironstones,  in  fact  all  the  phenomena  of  the  Carboniferous 


H  OIL-FINDING 

Coal  Measures  may  be  studied  in  Miocene  and  Pliocene  strata 
under  the  simplest  conditions.  The  higher  Tertiary  strata  on 
the  eastern  and  southern  and  western  coasts  of  Trinidad, 
where  excellent  and  almost  continuous  cliff  sections  can  be 
studied  and  mapped,  afford  perhaps  the  finest  examples  in  the 
world  of  deltaic  conditions  on  the  margin  of  a  Tertiary 
continent. 

One  very  instructive  section  in  the  Sangre  Grande  Ward 
may  be  instanced  here.  An  abundant,  though  not  very  rich, 
fauna  of  fresh  or  brackish  water  mollusca  occurs  in  a  grey 
littoral  sand,  which  also  contains  the  remains  of  twigs  and  leaves. 
On  the  one  side  this  sand  bed  passes  gradually  into  fine  sands 
and  silts  undoubtedly  marine  in  origin,  and  on  the  other  side 
into  carbonaceous  shales  with  strings  and  thin  beds  of  lignite. 
The  fossils  are  undisturbed  as  they  lived,  the  lamellibranchs 
having  both  valves  joined  and  closed.  It  is  quite  evident  that 
we  have  here  the  sand,  bank  at  the  mouth  of  a  lagoon,  littoral 
and  marine  conditions  on  the  one  hand,  and  freshwater  or 
terrestrial  conditions  on  the  other.  The  whole  section  is  not 
more  than  200  yards  in  extent,  and  as  the  beds  lie  almost 
horizontally  there  can  be  no  mistake  as  to  these  different 
conditions  existing  at  the  same  horizon. 

Evidence  such  as  this  leaves  no  room  for  doubt  as  to  the 
manner  in  which  these  Tertiary  lignites  and  their  associated 
strata  have  accumulated.  Occasionally  the  evidence  is  so 
complete  that  the  course  of  a  Tertiary  river  near  its  mouth  can 
actually  be  traced  through  the  strata,  the  lagoons  on  both  sides 
of  it  roughly  demarcated,  and  the  sand  and  pebble  banks  that 
intervened  between  lagoons  and  sea  mapped.  It  is  even  possible 
by  a  study  of  the  effects  of  current  action  in  the  sand  banks  to 
determine  that  the  prevailing  wind  was,  as  it  is  now,  from  the 
east,  and  by  a  study  of  the  finer  sediments  to  prove  that  the 
climate  was  wet,  the  lagoons  liable  to  fresh  water  floods,  and, 
in  fact,  that  conditions  were  very  much  the  same  as  at  present. 

Now  let  us  turn  to  the  evidence  from  oilfields  to  ascertain 
under  what  conditions  the  strata  associated  or  impregnated 
with  petroleum  have  been  formed.  So  far  as  lithological 
evidence  goes,  the  strata  of  many  Tertiary  oilfields  are  exactly 
the  same  as  those  associated  with  coals  or  lignites ;  there  are 
the  same  rapid  alternations,  the  same  constantly  repeated 
minor  succession  of  clay  ("  underclay "  in  the  case  of  coal  or 


THE   ORIGIN   OF   PETROLEUM  15 

lignite)  followed  by  sands  sometimes  conglomeratic,  passing 
upwards  into  marine  silts  and  clays,  "  underclay "  again,  and 
so  on. 

Considered  in  detail  the  resemblance  is  as  striking.  It  is 
in  the  thick  littoral  sands,  which  overlie  the  lignite  seams,  that 
shell  banks  occur,  and  not  infrequently  ironstone  nodules  and 
concretions,  suggesting  that  a  concentration  of  iron  compounds 
under  current  action  may  have  taken  place.  Bands  of 
calcareous  sandstone,  the  "  hard  shells "  of  the  driller's  logs, 
occur  not  infrequently  in  these  sand  groups,  and  especially  at 
the  top  of  them.  These  represent  littoral  deposits  in  which 
there  was  sufficient  shell  sand  to  form  a  cement  for  the  whole 
bed ;  when  the  calcareous  material  is  insufficient  for  this  purpose 
it  occurs  in  round  or  disc-shaped  concretions  often  of  large  size 
and  distributed  in  more  or  less  regular  lines.  All  these 
phenomena  are  as  characteristic  of  the  carbonaceous  as  of  the 
petroliferous  phases. 

In  fact,  the  only  difference  is  that  in  th%  one  case  we  have 
abundant  evidence  of  vegetable  remains  in  underclays,  leaf 
beds,  carbonaceous  shales  and  sandstones,  fossil  tree-trunks, 
and  seams  of  lignite  or  coal,  while  in  the  oilfield  phase  not  a 
trace  of  vegetable  matter  is  observed,  but  the  porous  beds  are 
more  or  less  impregnated  with  oil  or  bitumen,  which  may  often 
be  seen  exuding  at  the  outcrops. 

The  next  point  to  be  noted  is  that  careful  stratigraphical 
mapping  has  proved  that  the  same  horizons  that  are  carbonaceous 
in  one  locality  are  petroliferous  in  another,  often  within  quite 
a  short  distance;  the  only  variation  being  that  bartds  of 
impervious  clay  are  sometimes  more  conspicuous  among,  and 
especially  above,  the  petroliferous  strata  than  among  or  above 
the  carbonaceous. 

This  point  has  been  established  over  very  wide  areas  in 
Burma,  Trinidad,  and  other  countries,  by  careful  geological 
mapping  on  the  scale  of  six  or  more  inches  to  the  mile,  and  the 
change  from  the  petroliferous  to  the  carbonaceous  phase  can 
in  some  cases  be  shown  to  take  place  within  three  hundred 
yards. 

First  of  all,  taking  evidence  on  a  large  scale,  Burma 
furnishes  an  excellent  field  for  study.  The  stratigraphical  and 
palaeontological  work  of  the  Geological  Staff  of  the  Burma  Oil 
Co.  has  proved  that  the  great  productions  of  oil  in  the 


16  OIL-FINDING 

Yenangyoung,  Yenankyat  and  Singu  Fields  are  all  obtained 
from  horizons  which  are  represented  by  the  Yaw  Sandstone 
Group  and  the  strata  immediately  overlying  it,  which  have  been 
mapped  and  examined  over  very  many  miles  of  their  outcrops 
in  the  foothills  of  the  Arakan  Yomas  to  the  west.  These  great 
arenaceous  groups  exhibit  on  their  outcrops  both  the  petro- 
liferous and  carbonaceous  phases,  the  latter  however  pre- 
dominating, and  the  remains  of  terrestrial  vegetation  are  very 
common  throughout.  Traced  eastwards  these  groups  are  found 
to  become  more  or  less  split  up  by  bands  of  clay  and  their  total 
thickness  perceptibly  diminishes,  while  the  petroliferous  phase 
replaces  the  carbonaceous. 

Similarly  in  other  countries  where  oil  and  coal  or  lignite 
occur  within  the  same  series,  the  same  phenomena  are  to  be 
observed,  and  it  is  possible  to  work  out  generally  the  provinces 
of  oil-belt  and  coal-belt  in  strata  of  the  same  horizon.  The 
Ghasij  Shales  in  Baluchistan  afford  a  striking  example ;  a 
bituminous  coal  is  worked  in  one  district,  while  another  is 
characterized  by  oil-shows  derived  from  the  same  strata. 

To  follow  this  enquiry  in  greater  detail,  particular  beds  may 
be  selected  and  mapped  till  the  actual  transition  between  the 
two  phases  is  observed.  In  Burma,  in  several  localities,  lignite 
beds  have  been  traced  into  oil-bearing  rocks  containing  no 
trace  of  vegetable  remains.  In  the  Yaw  valley,  about  fifteen 
miles  south-west  of  Pauk,  is  one  of  the  best  instances,  the 
lignitic  phase  being  completely  superseded  and  replaced  by  the 
petroliferous  within  a  distance  of  three  hundred  yards,  the 
outcrop  being  followed  easily  as  the  strata  dip  steeply,  and 
a  hard  sandstone  bed  enables  the  horizon  to  be  followed  without 
the  possibility  of  mistake.  In  this  case  the  oil  is  a  light  one 
with  a  paraffin  base  and  containing  a  high  percentage  of  solid 
paraffin. 

In  Trinidad  similar  detailed  evidence  as  to  the  equivalence 
of  lignite  seams  and  oils  of  asphaltic  base  is  not  far  to  seek. 
In  mapping  the  southern  coast  section  in  that  island  the 
author  came  on  a  series  of  lignites  and  lignitic  shales  inter- 
calated with  sandstones  near  Grande  Eiviere.  The  dip  of  the 
strata  is  vertical,  and  the  strike  coincides  generally  with  the 
coastline  which  consists  of  an  almost  continuous  cliff,  so  that 
the  following  of  horizons  was  a  matter  of  the  greatest  simplicity. 
Within  a  mile  to  the  westward  the  lignitic  phase  was  replaced 


THE   ORIGIN   OF   PETROLEUM  17 

by  the  petroliferous  phase  in  the  same  horizons,  the  overlying 
strata  becoming  at  the  same  time  slightly  more  argillaceous  on 
the  whole  through  the  thickening  of  intercalated  bands  of  clay. 
All  traces  of  .vegetable  remains  were  lost  before  the  Eio  Blanco 
was  reached,  and  seepages  of  oil  out  of  the  porous  beds  were 
observed  in  several  places.  This  point  is  important  as  it  has 
been  frequently  urged  as  an  argument  against  the  vegetable- 
origin  theory  that  traces  of  vegetable  organisms  are  not  found 
among  oil-bearing  strata.  From  this  section  on  the  southern 
coast  the  name  Rio  Blanco  Oilbearing  Group  was  given  to  the 
strata  at  this  horizon  in  the  Tertiary  series,  an  horizon  which 
has  been  proved  to  be  not  only  petroliferous,  but  very  richly 
productive  in  many  parts  of  the  island.  In  only  two  other 
localities,  and  these  widely  separated,  are  lignitic  strata  known 
to  occur  at  this  horizon.  It  is  noticeable  that  where  the 
petroliferous  phase  is  in  evidence  impervious  clays  of  greater  or 
less  thickness  directly  overlie,  or  are  observed  at  no  great 
distance  above,  the  oil-bearing  sands. 

Still  more  remarkable  evidence  is  furnished  by  the  so-called 
"  porcellanites "  of  the  western  and  southern  districts  in 
Trinidad.  These  are  naturally  burnt  shales  which  have  become 
ignited  spontaneously,  just  as  the  bituminous  Kimeridge  clay 
in  coast-sections  in  the  south  of  England  has  done  on  many 
occasions.  The  brick-red  burnt  outcrops  of  these  porcellanites 
make  very  striking  features  in  the  scenery  of  the  Wards  of 
Oropuche  and  Cedros,  a  thickness  of  as  much  as  thirty  feet  of 
strata  exposed  in  a  cliff  being  sometimes  burnt  and  indurated. 

When  examined  closely  many  of  these  porcellanites  are 
found  to  be  leaf  beds,  being  a  mass  of  beautifully-preserved 
leaf  impressions ;  the  leaves  are  those  of  ordinary  terrestrial 
vegetation,  similar,  if  not  actually  belonging,  to  the  same  flora 
that  flourishes  at  the  present  day  in  the  colony.  Veins  and 
strings  of  clinker  are  seen  ramifying  through  the  masses  of 
burnt  shale,  and  occasionally  some  remains  of  sticky  asphalt 
may  be  observed  lining  joints  in  indurated  but  not  oxidised 
portions  of  the  outcrop. 

In  several  cases  lignitic  seams  are  observed  not  far  below 
an  outcrop  of  porcellanite,  and  Messrs.  Wall  &  Sawkins  in 
their  Memoir  upon  the  Geology  of  Trinidad  state  that  the  com- 
bustion of  lignite  has  burnt  the  overlying  shales.  The  author, 
however,  has  found  no  evidence  for  this ;  he  has  never  observed 

c 


i8  OIL-FINDING 

a  naturally  burnt  lignite.  It  is  the  shales  themselves,  either 
bituminous  or  in  a  transition  stage  between  bituminous  and 
carbonaceous,  that  have  ignited  spontaneously  and  burnt.  The 
ignition  is  probably  due  to  the  heat  engendered  by  the  oxida- 
tion of  sulphides,  as  in  the  case  of  the  Kimeridge  clay  ;  every 
stream  draining  a  porcellauite  outcrop  is  beautifully  clear,  and 
the  cause  of  this  is  soon  ascertained  when  the  water  is  tasted. 
It  is  full  of  alum,  proving  the  oxidation  of  sulphides. 

One  very  important  point  in  connection  with  these  porcel- 
lanites remains  to  be  mentioned.  Their  dip  is  always  very 
gentle,  and  though  the  outcrops  may  be  traced  for  miles  along 
the  coast  or  through  the  forest,  no  case  of  a  steeply  dipping 
porcellanite  has  been  observed. 

The  relation  of  porcellanites  to  oil-bearing  strata  is  interest- 
ing and  carries  us  a  step  further  in  the  enquiry  as  to  the  cause 
of  the  phenomenon.  The  lower  part  of  the  cover-clay  of  the 
La  Brea  Oil-bearing  Group  is  burnt  to  a  typical  porcellanite 
for  a  distance  of  nearly  two  miles  along  its  outcrop.  In  this 
case  leaf  impressions  are  absent  or  very  rare.  Some  patches 
along  the  outcrop  may  be  seen  occasionally  smouldering  at  the 
present  day ;  Professor  Cadman  has  photographed  a  smoulder- 
ing outcrop  near  La  Brea  (Plate  III).  Other  occurrences  of 
the  burning  of  the  clay  above  an  outcropping  oilsand  may  be 
seen  in  the  forest  south  of  Siparia  and  in  Burnt  Cliff  in 
Barbados,  where  a  petroliferous  shale  has  been  burnt  along 
the  outcrop  for  a  considerable  distance. 

.  This  establishes  the  fact  that  these  porcellanites  are  as 
much  associated  with  the  petroliferous  phase  as  with  the 
'carbonaceous.  It  might  be  suggested,  indeed,  that  porcellanite 
is  more  characteristic  of  a  petroliferous  than  of  a  lignitic  series, 
were  it  not  that  leaf-beds  are  essentially  phenomena  of  the 
carbonaceous  phase ;  while  the  occurrence  of  porcellanites  in  a 
lignitic  series,  where  no  signs  of  petroleum  have  been  detected, 
is  very  frequent,  e.g.  on  the  southern  coast  of  Trinidad  near 
Chatham  and  in  the  eastern  Lignite  District  near  Sangre 
Grande.  In  both  these  localities  thick  beds  of  the  porcellanite 
have  been  traced  for  miles  where  no  oilsand  is  known  to  occur, 
but  where  lignites  are  common. 

One  coast  section  west  of  Irois  is  specially  significant 
(Fig.  1).  A  porcellanite  outcrop  is  seen  dipping  at  an  angle 
of  some  three  or  four  degrees  at  right  angles  to  the  coast  line, 


Photo,  by  Prof.  J.  Cadman. 

i.  SMOULDERING  OUTCROP  NEAR  LA  BREA,  TRINIDAD. 


** 


"•j"NPri      :f*     *"  *•*      ir**j«r  •  -  *F^air^5WS.1'  - 

'Vv^&cv  '  •••r:-       /> 
--'T  ^?/>^'<  :.,•   v^ 


ii.  MUD-VOLCANO  IN  ERUPTION,  TRINIDAD. 


THE    ORIGIN    OF   PETROLEUM  19 

and  covered  by  argillaceous  beds.  This  dip  brings  the  out- 
crop below  tide  mark  where  the  strata  become  horizontal.  At 
a  distance  of  less  than  one  hundred  yards  the  strata  emerge 
with  a  low  dip  in  the  opposite  direction,  thus  forming  a  very 
gentle  local  syncline.  Where  the  strata  emerge  the  argillaceous 
beds  have  thinned  out  or  become  replaced  by  arenaceous  strata, 
and  the  beds  beneath  are  no  longer  burnt  but  consist  of 


FIG.  1. — Coast  section  west  of  Irois' (Trinidad).     (Length  about  200  yds.) 
1.  Porcellanite ;  2.  Clay;  3.  Impure  lignite  and  shale  ;  4.  Sandstone. 

carbonaceous  shales  with  one  seam  of  impure  lignite.  This 
section  can  be  observed  from  the  local  gulf  steamer  on  its  daily 
route  from  San  Fernando  to  Cedros  and  back,  and  can  be  studied 
in  detail  during  a  walk  along  the  beach.  The  whole  section  is 
some  two  hundred  yards  in  length,  and  is  so  well  exposed  that 
there  is  no  possibility  of  misunderstanding. 

Such  evidence  places  beyond  doubt  the  connection  between 
the  lignitic  and  petroliferous  phases  of  these  Tertiary  strata, 
and  emphasises  once  again  the  point  that  a  slight  difference  in 
environment,  the  change  from  an  arenaceous,  that  is  to  say,  a 
porous,  cover  to  an  argillaceous  or  impervious  cover,  seems  to 
determine  whether  the  strata  have  ignited  and  burnt  to  porcel- 
lanites  or  have  remained  as  unburnt  lignitic  shales.  It  is 
obvious  that  where  strata  lie  at  low  angles  the  presence  of  an 
impervious  cover  will  tend  to  preserve  any  combustible  or 
volatile  matter  that  may  be  in  evidence  in  the  underlying 
strata  from  being  rapidly  dissipated  or  removed  by  weathering, 
and  thus  will  favour  a  slow  combustion  if  a  temperature  suffi- 
cient to  cause  ignition  be  reached. 

We  may  safely  conclude,  then,  that  these  "  porcellanites " 
of  Trinidad  represent  a  transition  stage  between  the  purely 
petroliferous  and  the  purely  carbonaceous  phases,  they  have 
been  more  or  less  bituminous  shales,  and  to  attribute  their 
combustible  matter  to  an  animal  origin  would  be  the  most 
unjustifiable  of  assumptions. 

The  above  evidence,  selected  from  a  mass  of  similar  details. 


20  OIL-FINDING 

is  sufficient  to  prove  that  in  known  oilfields  the  equivalence  of 
lignitic  and  petroliferous  beds  under  slightly  varying  conditions 
is  indisputable.  It  remains  now  to  show  that  in  known  coal- 
fields, the  association  of  petroleum  with  carbonaceous  strata  is, 
though  perhaps  rare,  by  no  means  unprecedented.  The  point 
to  be  considered  is  the  environment,  the  conditions  to  which  the 
vegetable  matter  has  been  subjected.  There  are  very  many 
instances  on  record  of  a  series  being  petroliferous  in  the  lower 
beds,  and  lignitic  or  coal-bearing  in  the  upper  members.  In 
such  cases  it  will  always  be  found  that  a  greater  or  less 
thickness  of  comparatively  impervious  strata  intervenes  between 
the  two  phases.  The  section  at  Point  Ligoure  on  the  Western 
coast  of  Trinidad  shows  this  very  clearly,  while  in  Borneo, 
Eussia,  West  Virginia,  and  many  other  countries,  lignite  or 
coal  characterizes  the  less  loaded  or  less  perfectly  sealed 
horizons  of  a  series.  In  the  latter  country  oil  wells  are  some- 
times drilled  through  workable  coal-seams,  and  the  bores  have 
to  be  cased  carefully  to  prevent  water  entering  the  coal-seams 
and  flooding  the  coal  workings. 

The  evidence  as  to  environment  is  confirmed  by  recent 
researches  on  the  nature  of  coals,  and  the  conditions  under 
which  they  are  found.  It  was  until  recently  the  accepted  view 
that  anthracites  are  characteristic  of  the  most  disturbed, 
contorted,  and  faulted  parts  of  a  coalfield,  and  that  bituminous 
coals  are  characteristic  of  the  less  disturbed  portions.  This 
theory,  though  there  seemed  at  one  time  to  be  ample  evidence 
for  it,  can  no  longer  be  held  owing  to  the  careful  researches  of 
H.M.  Geological  Survey  in  the  South  Wales  and  Staffordshire 
coalfields.  Anthracites  occur  in  comparatively  slightly  disturbed 
strata  in  South  Wales,  and  bituminous  coal  at  the  same  horizons 
in  less  disturbed  areas.  It  has  been  suggested  that  differences 
in  the  original  conditions  of  deposition  may  account  for  this, 
particularly  as  the  amount  of  ash — inorganic  material  included 
in  the  coal — varies  at  the  same  time,  being  greater  in  the 
bituminous  coals.  Probably  a  truer  explanation  is  to  be  sought 
for  in  the  fact  that  the  anthracites  occur  where  the  coal  has 
been  open  to  the  influence  of  deep-seated  weathering,  and 
where  the  structure  and  nature  of  the  covering  have  favoured 
the  loss  of  volatile  constituents.  The  greater  original  purity 
of  the  deposit  is  also  a  factor  to  be  reckoned  with  ;  a  pure  coal 
will  readily  give  up  its  volatile  or  bituminous  contents,  while 


THE   ORIGIN   OF   PETROLEUM  21 

an  impure  coal,  owing  to  the  "  adsorptive  "  capacity  of  finely 
divided  inorganic  matter  for  bitumen,  retains  it  to  a  much 
greater  extent  and  will  not  part  with  it  altogether,  even  under 
the  action  of  organic  solvents. 

Thus  it  is  in  deep  mines  where  the  seams  do  not  crop  out 
at  the  surface,  but  are  well  sealed  up  beneath  impervious 
strata,  no  matter  how  contorted,  that  we  must  look  for  evidence 
of  petroleum.  And  the  evidence,  though  somewhat  scanty  at 
present,  and  unfortunately  not  always  recorded,  is  not  wanting. 
Miners  who  have  worked  in  deep  workings  of  bituminous  coal 
tell  of  "  tarry  oozings  "  from  the  neighbourhood  of  seams,  or 
along  joints  in  hard  close-grained  strata.  Quite  recently  a 
seepage  of  petroleum  has  been  recorded  from  the  Sovereign  Pit 
of  the  Wigan  Coal  and  Iron  Co.,  at  Leigh,  Lancashire,  at  a 
depth  of  some  600  yards. 

The  oil-shales  of  Scotland,  though  not  oilrocks  in  the  strict 
sense,  add  their  quota  to  the  mass  of  evidence  connecting 
petroleum  and  coal.  These  shales  are  contemporaneous  with 
coal-bearing  strata,  and  though  they  have  never,  perhaps,  been 
actually  petroliferous,  they  by  their  nature  have  been  enabled 
to  retain  in  great  quantity  the  material  which  under  different 
conditions  would  have  been  vast  stores  of  liquid  hydrocarbons. 
The  association  of  coal  seams  and  oil-shale  beds  is  so  frequent, 
e.g.  in  the  Wolgan  Valley  in  Australia,  that  it  is  unnecessary 
to  enlarge  upon  this  point. 

Thus  we  see  that  though  coal  and  lignite  are  very  different 
substances  from  liquid  petroleum,  they  are  inextricably  con- 
nected ;  coalfields  give  evidence  of  oil  and  oilfields  of  coal, 
transitional  stages  can  be  searched  for  and  found,  and  both 
asphaltic  and  paraffin  oils  are  seen  impregnating  the  same 
strata  which  at  no  great  distance  are  carbonaceous  in  character. 
Such  evidence,  almost  always  forthcoming  as  the  result  of 
careful  and  detailed  stratigraphical  work  in  any  part  of  the 
world  where  petroleum  is  to  be  found,  makes  it  hardly 
possible  to  doubt  that  it  is  to  terrestrial  vegetation  that  we 
must  look  for  the  raw  material  from  which  our  supplies  of 
petroleum  are  derived. 

But  while  stating  this  conclusion  it  must  be  borne  in  mind 
that  in  certain  cases,  as  for  instance  gas  coals  and  oil- shales, 
it  is  quite  probable  that  such  animal  matter  as  may  have  been 
preserved  has  borne  its,  very  minor,  part.  The  ammonia 


22  OIL-FINDING 

derivable  from  a  gas  coal  or  some  of  the  oil-shales  would 
certainly  suggest  that  some  animal  matter  may  have  been 
present.  But  all  oil-shales  do  not  contain  ammonia  in  com- 
bination, and  petroleum  is  almost  entirely  free  from  nitrogenous 
compounds,  so  that  we  may  regard  the  ammonia  contents 
rather  as  adventitious  than  as  essential.  The  Kimeridge  clay 
would  certainly  be  mined  as  an  oil-shale  and  distilled,  were 
it  not  for  the  absence  of  ammonia,  which,  fixed  as  the  sulphate, 
is  the  most  valuable  by-product  of  the  Scottish  oil-shales. 

B.  (2)  Vegetable  Origin. — Before  leaving  this  branch  of  the 
subject,  it  is  necessary  to  refer  to  an  idea  or  hypothesis 
frequently  put  forward  in  a  rather  indefinite  manner,  but  which 
has  found  favour  with  many,  especially  those  who  have  little 
field  experience. 

This  hypothesis  is  that  petroleum  is  formed  from  marine 
vegetation ;  in  other  words  seaweeds  or  fucoids.  It  was 
apparently  the  desire  to  find  some  marine  origin  for  oil  that 
caused  this  theory  to  be  taken  up,  and  allusions  to  "  fucoids  " 
by  writers  on  the  subject  of  petroleum  were  at  one  time  very 
frequent. 

But  the  origin  of  the  theory  was  a  series  of  observations 
made  on  decomposing  seaweed  on  the  coasts  of  Sicily,  Sardinia, 
Norway,  and  other  countries,  where  a  "jelly-like  substance" 
was  found  to  be  formed  at  one  stage  of  the  subaerial  decom- 
position. This  "jelly-like "  matter  was  somewhat  loosely 
described  as  "  substances  resembling  petroleum,"  and  the 
theory  of  a  seaweed  origin  for  oil  sprang  to  birth  in  the  minds, 
not  of  the  observers  themselves,  but  of  others  who  read  about 
it.  The  theory  was  again  revived  by  the  discovery  of  "  Nhangel- 
lite  "  formed  in  Portuguese  South  Africa,  by  the  decomposition 
of  fresh  water  algaa  in  dried-up  shallow  lakes,  and  the  claim 
that  this  Nhangellite  was  evidence  of  the  existence  of  petroleum 
in  the  neighbourhood.  In  1906  the  author  was  asked  to 
investigate  this  claim  in  the  field,  but  the  evidence  seemed 
insufficient  to  justify  the  necessary  expenditure  of  time. 

So  far  as  geological  field  evidence  has  been  adduced  in 
favour  of  this  theory,  it  has  been  confined  to  the  production 
of  a  few  specimens  of  so-called  fucoids,  but  in  most  cases,  as 
in  the  famous  case  of  the  Cambrian  Fucoid  Beds  of  the  north- 
west of  Scotland,  examination  has  proved  the  so-called  "fucoids  " 
to  be  worm  tracks  and  burrows.  The  exposure  of  this  evidence, 


THE   ORIGIN    OF   PETROLEUM  23 

however,  has  not  entirely  removed  the  theory  from  currency : 
a  theory  can,  it  appears,  survive  the  loss  of  the  last  piece  of 
direct  evidence  in  its  favour. 

Let  us  again  appeal  to  the  facts  and  consider  what  evidence 
can  be  brought  up  for  and  against  a  seaweed  origin  for  petroleum. 
In  the  first  place,  are  there  any  inherent  improbabilities  in  the 
theory  ?  Is  it  possible  for  seaweed  to  be  accumulated  in  vast 
quantities  and  entombed  in  sediments  as  they  are  deposited  ? 
That  vast  quantities  would  be  required  will  be  admitted,  as  by 
far  the  greater  part  by  weight  of  seaweed,  about  seven-eighths, 
is  water. 

Under  what  conditions  do  seaweeds  flourish  most  luxuri- 
antly ?  It  is  a  simple  matter  of  observation.  On  rocky  coasts, 
in  comparatively  clear  water,  and  in  stagnant  marine  areas 
such  as  the  Sargasso  Sea,  seaweed  can  grow  abundantly.  But 
in  neither  case  is  there  any  probability  of  the  seaweed  sinking 
and  becoming  entombed  in  sediment.  On  rocky  coasts  the 
weed  is  torn  off  by  storms  and  cast  on  the  shore,  e.g.  in  the 
west  of  Scotland  and  Ireland,  where  kelp-gathering  is  a  regular 
industry.  In  the  Sargasso  Sea  the  weed  is  floating  or  attached 
to  floating  timbers,  the  remains  of  derelicts,  etc. 

In  deep  water,  beyond  the  laminarian  zone,  seaweeds  a*e 
rare,  small,  and  insignificant.  In  muddy  estuaries,  under 
deltaic  conditions,  which  have  been  proved  to  be  the  environ- 
ment in  which  strata  now  oil-bearing  have  accumulated,  where 
in  fact  sedimentation  proceeds,  apace,  there  would  seem  to  be 
some  possibility  of  weeds  becoming  involved  and  preserved  in 
the  rapidly  forming  deposits,  but  in  such  conditions  the  waters 
are  singularly  free  from  seaweed  growth.  Thus  the  initial 
difficulty  of  postulating  the  possibility  of  a  sufficient  quantity 
of  raw  material  is,  perhaps,  even  greater  in  the  case  of  the 
marine- vegetation  theory  than  in  the  case  of  the  animal- origin 
theory. 

Turning  to  chemical  evidence,  there  are  facts  even  more 
difficult  to  explain  away.  When  the  water  is  removed  from 
seaweed,  of  the  remaining  solids  a  considerable  proportion  is 
bromine  and  iodine  in  the  form  of  salts.  In  fact,  it  is  from  the 
ash  of  seaweeds  that  these  elements  are  extracted  commercially. 
If  petroleum  is  formed  from  the  remains  of  seaweed,  what 
becomes  of  these  bromides  and  iodides  which  must  be  present 
in  enormous  quantity  ?  In  one  case  a  trace  of  iodine  has  been 


24  OIL-FINDING 

detected  in  the  water  from  a  mud- volcano,  but  the  proportion 
was  quite  insignificant  compared  with  the  trace  of  petroleum  in 
the  same  water. 

The  marine-vegetation  theorists  must  account  for  the  loss  or 
disappearance  of  these  salts  before  they  can  justify  the  chemical 
possibility  of  their  hypothesis.  Again,  practically  every 
sample  of  petroleum  that  has  ever  been  analysed  contains  some 
trace  of  sulphur,  and  the  percentage  rises  to  three  or  more  in 
some  cases.  But  there  is  no  sulphur  in  seaweeds. 

The  chemical  difficulties  to  be  surmounted  are  therefore  as 
insurmountable  as  the  initial  difficulty  of  accumulation  in 
sufficient  quantity. 

If  field  evidence  of  unimpeachable  character  were  available, 
the  matter  would  be  worthy  of  serious  consideration  ;  if  fucoids 
and  traces  of  fucoids  were  found  in  quantity  throughout  a 
series,  and  only  disappeared  among  strata  actually  petroliferous, 
it  would  be  necessary  to  give  special  attention  to  the  role 
played  by  that  class  of  organism  and  the  strata  in  which  the 
evidence  occurs,  but  when  most,  if  not  all,  of  the  so-called 
"fucoids"  are  worm-casts  and  tracks  of  animal  organisms,  the 
practical  geologist  is  unable  to  treat  the  theory  with  respect. 

Thus  every  hypothesis  but  that  of  the  origin  from  terrestrial 
vegetation  fails  when  tested  by  an  appeal  to  the  facts  to  be 
observed  at  the  present  day,  and  we  may  confidently  state  that 
the  only  source  of  origin  which  is  at  the  same  time  adequate 
and  within  the  bounds  of  chemical  and  physical  possibility  is 
terrestrial  vegetation. 


CHAPTER   II 

PBOCESSES   OF    FORMATION 

IN  the  last  chapter  we  have  dealt  with  the  material  from  which 
petroleum  is,  or  can  be,  formed,  and  the  various  theories  that 
have  been  put  forward  to  account  for  its  origin. 

It  now  becomes  expedient  to  consider  the  processes  through 
which  the  raw  material  must  pass  in  order  to  convert  it  into 
the  mixture  of  saturated  and  un saturated  hydrocarbons  which 
we  know  as  "crude  petroleum."  The  problem  is  to  find  out 
whai:  these  processes  are,  and  how  they  have  affected  the  raw 
material. 

A  simple  distillation  caused  by  heat  will  not  meet  the  case 
entirely.  We  have  seen  already  that  such  distillations  take 
place  in  nature  where  igneous  rocks  invade  coal  or  oil-shale 
measures.  Instances  of  this  are  frequent  among  the  Scottish 
oil-shales,  and  semi-liquid  bitumen  occurs  as  an  impregnation 
in  porous  strata  or  along  joints  and  in  cavities  for  some  distance 
from  the  shale  bed  or  from  the  intrusion.  But  the  result  is 
not  the  reproduction  of  an  oilfield  on  a  small  scale,  nor  could 
the  process  take  place  upon  a  sufficiently  large  scale. 

What  is  required  is  a  simple,  slow,  natural  process  which 
can  take  place  over  wide  areas.  It  is,  without  doubt,  more  in 
the  province  of  the  chemist  than  of  the  geologist  to  make 
investigations  with  the  view  of  determining  under  what  con- 
ditions in  nature  it  is  possible  to  form  petroleum  from  whatever 
raw  material  is  available;  but  the  geologist's  evidence  is 
necessary,  if  only  to  prevent  undue  attentkm  being  given  to 
entirely  artificial  conditions  which  may  be  arranged  for  in  the 
laboratory,  but  which  can  hardly  be  reproduced  in  nature. 

Many  chemists  have  conducted  researches  upon  petroleum 
with  a  view  to  proving  its  mode  of  origin  and  the  processes 
necessary  for  its  formation,  and  no  more  careful  and  interesting 
work  has  been  done  than  by  Engler  and  Hofer.  These  observers 

25 


26  OIL-FINDING 

state  very  clearly  the  conditions  under  which  the  reactions 
they  observed  and  controlled  took  placo,  and .  the  care  and 
accuracy  of  their  researches  cannot  be  doubted.  But  they  do 
not — and  the  same  objection  applies  to  the  work  of  many 
others  on  the  same  subject — approach  the  inquiry  from  the 
point  of  view  as  to  what  conditions  are  possible  in  nature, 
conditions  which  the  geologist  in  however  rough  a  manner  is 
able  to  define.  Thus  the  work  of  these  scientists,  careful  and 
painstaking  as  it  is,  is  open  to  the  charge  of  what  might  be 
called  a  form  of  special  pleading  in  experimental  work.  Given 
the  conditions  they  postulate,  the  results  are  certain,  but  if 
such  conditions  are  practically  impossible  on  a  large  scale  in 
nature,  the  researches  conducted  in  a  laboratory  become  of 
little  value  to  the  practical  man  whose  business  is  to  find  oil. 

The  geologist  from  his  observation  of  the  conditions  under 
which  petroleum  occurs,  knows  the  conditions  to  which  the 
series  of  strata  containing  petroleum  must  have  been  subjected. 
Some  universal  process,  subject  to  these  conditions,  is  called 
for,  and  it  is  the  duty  of  the  chemist  rather  than  of  the  geologist 
to  reproduce  as  far  as  is  possible  the  conditions  so  defined,  and 
to  prove  whether  it  is  possible  to  form  the  mixture  of  hydro- 
carbons known  as  crude  petroleum  from  the  raw  material 
supplied  and  under  the  stipulated  conditions. 

Now  the  only  conditions  which  the  geologist  has  any  right 
to  dogmatise  about  are  depth-temperature,  pressure,  the  presence, 
or  absence  of  water,  the  nature  of  the  raw  material,  and  the 
question  as  to  whether  or  not  the  strata  in  which  the  chemical 
reactions  take  place  have  been  sealed  and  isolated  from  the 
introduction  of  extraneous  material. 

In  the  last  chapter  the  nature  of  the  raw  material  has  been 
discussed  at  length,  and,  so  far  as  is  possible  at  present, 
determined.  The  calculation  of  depth- temperature  is  simple, 
and  within  reasonable  limits  the  temperature  at  which  oil  may 
be  formed  can  be  deduced  from  incontrovertible  evidence.  The 
calculation  of  pressure  is  a  matter  of  much  greater  difficulty, 
and  there  must  necessarily  be  a  very  wide  range  between  the 
minimum  and  maximum  pressures  postulated.  The  sealing 
up  of  the  strata,  in  other  words  the  determination  as  to  whether 
the  reactions  have  taken  place  in  open  or  closed  retort,  is  again 
a  matter  of  easy  determination,  seeing  that  it  is  admitted  by  all 
observers  that  for  the  formation  or  preservation  of  oil  impervious 


PROCESSES    OF   FORMATION  27 

strata  must  overlie  the  petroliferous  rocks.  Similarly  the 
presence  or  absence  of  water,  argillaceous  material,  sodium 
chloride,  and  other  material  either  active  or  inert  in  the 
chemical  sense,  can  be  deduced  with  a  fair  degree  of  certainty. 

Here  we  must  turn  to  the  laboratory  to  learn  what  ex- 
perimental investigations  will  come  to  our  aid ;  it  is  a  question 
of  conditions  favourable  to  chemical  reaction. 

It  has  been  stated  that  wood  sealed  in  a  closed  tube  with  a 
small  quantity  of  water  and  subjected  to  great  pressure  at 
ordinary  temperatures  has  produced  a  small  quantity  of  mixed 
hydrocarbons  analogous  to  a  crude  petroleum,  but  I  have  been 
unable  to  verify  this  interesting  result  or  to  obtaiji  any  details 
about  the  experiment.  The  various  attempts,  however,  to  make 
commercial  use  of  peat- masses  furnish  us  with  valuable  evidence. 
In  Ireland,  Sweden,  the  United  States,  and  other  countries,  the 
problem  of  how  to  utilize  the  enormous  accumulations  of  peat 
has  for  many  years  occupied  the  attention  of  practical  chemists 
and  chemical  engineers,  and  after  many  failures  it  seems  'that 
some  of  the  processes  are  within  sight  of  commercial  success. 
Without  disclosing  information  confidentially  received  it  may 
be  stated  that  all  these  processes  have  this  in  common,  that 
the  peat  after  being  dried  and  perhaps  ground  and  again 
pressed  into  briquettes,  is  subjected  to  destructive  distillation 
in  the  presence  of  a  limited  quantity  of  water,  under  greatly 
pressure,  and  at  a  comparatively  low  temperature. 

The  resulting  products  are  various  according  to  the  end 
aimed  at  and  the  different  pressures  and  temperatures  in  each 
case.  Bituminous  compounds,  petroleum  of  almost  every  grade,  " 
and  even  coke  may  be  obtained,  while  ammonium  salts  may  be 
recovered  as  sulphate  by  a  process  similar  to  that  used  in  the 
oil-shale  and  gas  industries. 

The  important  points  for  the   geologist  to  note    are  that 
petroleum  of  various  grades  and   in  great   quantity   can   be 
produced,  and  that  the  essential  conditions  are  great  pressure,-- 
cornparatively  low  temperature,  and  the  presence  of  a  limited  ^ 
quantity  of  water. 

Water  is  in  any  case  present  in  the  peat,  even  after  drying, 
for  it  is  as  impossible,  without  destructive  distillation,  to 
remove  the  combined  water  in  peat  as  it  is  in  the  case  of  a 
lignite. 

It  is  obvious  that  similar  conditions  can  easily  be  obtained 


28  OIL-FINDING 

in  nature.  The  presence  of  water  in  greater  or  less  quantity 
is  almost  inevitable  in  sedimentary  rocks,  the  requisite  pressure 
is  amply  provided  for  by  a  covering  of  a  few  hundred,  or  it 
may  be  thousand,  feet  of  superincumbent  strata,  while  as  soon 
as  decomposition  commences '  the  potential  gas  pressure  may 
become  so  great  that  almost  any  hydrostatic  pressure  required 
can  be  obtained.  The  temperature,  increasing  as  it  does  on  a 
general  average  one  degree  Fahrenheit  for  every  55  feet  of 
descent  into  the  earth's  crust  after  the  first  hundred,  would 
soon  be  raised  sufficiently  to  favour  chemical  reaction,  while  as 
pressure  increased  the  temperature  would  also  rise  till  the 
necessary  equilibrium  was  reached.  Thus  once  the  process  of 
petroleum  formation  has  commenced,  its  action  is  probably 
automatic  and  must  be  complete,  unless  there  is  a  change  in 
conditions.  The  sealing  up  of  the  strata  by  impervious  rocks, 
so  that  escape  of  gaseous  or  volatile  compounds  is  entirely 
prevented  or  rendered  so  slow  and  gradual,  as  to  be  quite 
insignificant,  is,  as  has  already  been  stated,  a  question  upon 
which  there  is  a  general  consensus  of  opinion. 

It  seems  probable — but  here  we  enter  into  speculation— 
that  it  is  the  pressure  that  is  the  determining  factor,  as  it  is  in 
so  many  chemical  reactions.  Given  the  vegetable  matter  from 
which  petroleum  can  be  formed  enclosed  in  a  well-sealed 
deposit,  given  the  presence  of  a  limited  quantity  of  water,  and 
the  necessary,  but  by  no  means  high,  temperature,  as  soon  as 
the  pressure  reaches  a  certain  point  the  action  will  begin.  In 
a  deltaic  area  undergoing  earth-movement,  as  is  almost  invari- 
ably the  case  on  the  margin  of  a  continent,  sediment  accumulates 
very  rapidly.  A  geosynclinal  on  a  large  or  small  scale,  in  fact, 
is  formed,  and  though  sedimentation  may  occasionally  outstrip 
subsidence,  or  subsidence  outstrip  sedimentation,  the  general 
result  is  the  growth  of  the  deltaic  deposits  outwards  by  pro- 
gressive sedimentation  over  a  continually  increasing  thickness 
of  strata  belonging  to  the  same  series.  In  such  circumstances 
the  requisite  pressure  for  the  formation  of  petroleum  may 
easily  be  obtained  in  the  strata  sufficiently  deeply  buried. 

Another  probable  effect  of  pressure  also  must  be  con- 
sidered ;  ceteris  paribus,  the  quality  of  the  petroleum  formed 
is  likely  to  depend  upon  it.  In  the  process  of  isomerisation 
of  organic  compounds,  it  has  been  proved  over  and  over  again 
in  the  laboratory  that  pressure  is  usually  the  determining 


PROCESSES    OF   FORMATION  29 

factor.  Thus  a  higher  pressure  may  determine  a  more  com- 
plete condensation  of  the  volatile  compounds  and  gases  into 
light  oils,  provided  that  such  condensation  is  accompanied  by 
a  decrease  in  total  volume.  The  fact  that  in  many  oilfields 
where  several  separate  sands  at  different  depths  contain 
petroleum,  the  specific  gravity  of  the  oil  generally  decreases 
as  the  depth  increases  may  not  be  due  in  all  cases,  as  has 
often  been  assumed,  entirely  to  partial  and  progressive  inspis- 
sation  of  the  shallow  oils,  but  partly  to  the  pressure  under 
which  the  petroleum  has  in  each  case  been  formed. 

On  this  hypothesis  of  oil- formation  the  importance  of  an 
impervious  "  cover  "  also  becomes  apparent.  The  "  cover  "  is 
in  effect  the  lid  of  the  retort  in  which  the  chemical  processes 
take  place.  If  the  lid  be  imperfect  or  imperfectly  closed, 
escape  of  gaseous  products  must  ensue,  pressure  can  never 
become  very  high,  and  the  entire  process  of  oil-formation  may 
be  prevented,  arrested,  or  permanently  stopped.  Coals  or 
lignites  and  carbonaceous  shales  and  sandstones  will  be  the 
result.  This  accounts  for  the  occurrence  of  porcellanite  beneath 
or  forming  part  of  a  bed  of  shale  or  clay,  while  the  lignitic 
or  carbonaceous  phase  is  in  evidence  where  the  cover  is 
arenaceous  and  porous. 

It  has  been  suggested,  on  account  of  the  association  of  oil- 
bearing  rocks  with  clays  or  shales  often  of  great  thickness,  that 
the  argillaceous  strata  may  have  had  some  actual  part  in  the 
formation  of  the  petroleum.  This  is  a  point  very  difficult  of 
proof,  either  for  or  against,  since  to  bring  actual  evidence  of  the 
favouring  of  chemical  action  by  the  presence  of  argillaceous 
material  which  itself  remains  unaffected  is  well-nigh  impossible. 
It  is  quite  probable  that  much  of  the  material  from  which 
petroleum  is  formed  has  been  deposited  with  and  included  in 
argillaceous  sediment,  witness  the  leaf  beds  which  have  been 
burnt  at  outcrop  to  porcellanites.  It  is  also  certain,  as  proved 
by  Mr.  Clifford  Richardson,  that  clays  can  absorb  and  "  adsorb  " 
bitumen  to  a  remarkable  extent,  and  can  be  used  to  filter 
solutions  of  asphalt  and  asphaltic  oils.  But  these  facts  are  not 
proofs  of  the  argillaceous  material  taking  any  actual  part  in 
the  chemical  processes  by  which  oil  is  formed,  even  as  what 
used  to  be  called  a  "  carrier,"  a  compound  which,  though  itself 
apparently  unaltered,  enables  chemical  action  to  take  place  by 
continual  decomposition  and  simultaneous  re-formation.  It  is 


30  OIL-FINDING 

an  interesting  field  for  research  for  chemists  to  enquire  into  the 
possibility  of  argillaceous  strata  having  some  such  essential 
role  to  play.  For  the  geologist  the  matter  of  importance  is 
simply  that  potential  oil-bearing  strata  require  an  impervious 
cover  if  the  oil  is  to  be  formed,  and,  when  formed,  if  it  is  to 
be  preserved  from  inspissation,  and  that  argillaceous  rocks, 
especially  fine  marine  and  estuarine  clays  and  shales  are  the 
best  and  most  usual  "  cover-rocks." 

By  studying  the  subject  of  pressures  in  the  earth's  crust, 
and  by  careful  measurement  of  sections  where  oil-bearing  strata 
are  exposed,  it  may  be  possible  to  arrive  at  some  idea  of  the 
pressure  necessary  for  the  formation  of  petroleum.  In  many 
cases  where  large  thicknesses  of  strata  are  exposed  it  will  be 
found  that  the  lower  part  of  the  series  is  petroliferous  and  the 
upper  part  carbonaceous,  without  there  being  any  essential 
change  in  the  character  of  the  intercalated  sediments  associated 
with  the  oil-bearing  and  lignitic  bands.  It  may  be  that  the 
upper  part  of  the  series  has  never  been  under  sufficient  pressure 
to  bring  about  petroleum-forming  reactions. 

Let  us  take  a  specific  case  and  attempt,  however  roughly, 
to  calculate  the  maximum  and  minimum  pressures  which  can 
have  been  exerted  during  the  formation  of  the  petroleum.  At 
Point  Ligoure  on  the  western  coast  of  Trinidad,  where  the 
Guapo  Oil  Company  is  operating,  there  is  a  very  clear  section 
exposing  some  1300  feet  of  strata,  the  dip  varying  from 
vertical  at  the  northern  and  lower  end  of  the  section  to  56 
degrees  at  the  southern  and  upper  end.  The  lower  600  feet  are 
in  the  petroliferous  phase,  and  several  bands  of  oil-rock  are 
exposed  especially  near  the  base  of  the  section.  In  the  upper 
200  feet  of  the  section  lignitic  clays  and  sands  with  underclays 
and  thin  seams  of  lignite  are  observed.  In  the  lower  part  of 
the  section  the  strata  are  somewhat  more  highly  mineralized, 
concretions  chiefly  cemented  with  iron-salts  are  more  frequent, 
and  there  are  several  beds  of  fairly  stiff  argillaceous  material 
intercalated  with  the  oil-bearing  sandstones  and  above  them. 
In  this  case  the  mapping  of  the  neighbouring  districts  has 
proved  that  probably  not  more  than  800  to  900  feet  of  strata 
have  ever  been  deposited  above  the  uppermost  beds  in  the 
measured  section.  Assuming  that  such  a  total  thickness  of  beds 
has  been  deposited  in  a  horizontal  position,  and  again,  assuming 
that  the  pressure  can  be  calculated  as  a  hydrostatic  pressure 


PROCESSES   OF   FORMATION  31 

directly  due  to  the  weight  of  the  superincumbent  strata — these 
being  great,  and  perhaps  hardly  justifiable  assumptions — it  is 
possible  to  calculate  the  pressure  to  which  the  strata  containing 
the  raw  material  from  which  petroleum  can  be  produced  have 
been  subject. 

Taking  the  specific  gravity  of  the  strata  to  be  on  an  average 
2*7,  we  arrive  at  the  result  that  the  maximum  pressure  exerted 
and  applied  in  this  instance  has  been  189  atmospheres,  or  some 
one-and-a-quarter  tons  per  square  inch,  and  the  minimum 
approximately  135  atmospheres  or  rather  less  than  a  ton  per 
square  inch  on  the  strata  now  found  to  be  oil-bearing,  while 
a  pressure  of  99  atmospheres  was  apparently  insufficient  to 
determine  the  formation  of  petroleum.  This  calculation  is,  of 
course,  open  to  many  sources  of  error,  and  it  is  improbable  that 
such  high  pressures  have  been  exerted  in  this  case,  as  earth- 
movement  and  denudation  probably  prevented  the  accumulation 
of  any  such  thickness  of  strata  in  a  horizontal  position.  The 
figures  are  only  given  to  suggest  a  form  of  enquiry  in  which 
the  observation  of  facts  in  the  field  may  enable  the  geologist  to 
obtain  evidence  as  to  the  conditions  requisite  for  the  formation 
of  petroleum.  In  this  case  the  oil,  as  yielded  at  present,  is 
of  fairly  high  gravity  with  an  asphaltic  base.  Another  instance 
may  be  cited  from  a  different  region.  In  the  valley  of  the 
Yaw,  in  Upper  Burma,  an  excellent  section  through  the  entire 
Pegu  Series  of  Burma  may  be  studied,  the  total  thickness  being 
some  8000  feet.  The  lower  3000  feet  exhibit  here  and  there 
evidence  of  the  petroliferous  phase  in  seepages  of  a  fairly  light 
oil  with  paraffin  base,  but  lignitic  beds  begin  to  appear  on  the 
same  horizons  as  the  oil-bearing  rocks  at  about  3000  feet  above 
the  base  of  the  series.  Then,  after  passing  upwards  through 
some  1300  to  1400  feet  of  strata  chiefly  of  solid  clays,  the 
lignitic  phase  is  well  represented  by  a  series  of  seams  with 
intervening  underclays  and  sandstones,  and  up  to  the  top  of  the 
section  no  further  evidence  of  petroleum  is  forthcoming.  In 
this  case  it  is  practically  certain  that  earth-movement  had 
begun  long  before  the  deposition  of  the  higher  beds,  and  that 
tho  strata  were  never  superimposed  upon  each  other  in  a 
horizontal  position.  Thus  calculations  of  pressure  and  tempera- 
ture from  the  data  as  given  might  be  entirely  erroneous.  The 
points  to  be  noted,  however,  are  that  a  transition  from  the 
petroliferous  to  the  carbonaceous  phases  takes  place  at  a  fairly 


32  OIL-FINDING 

definite  horizon  in  the  series,  and  that  this  change  may  not  be 
due  entirely  to  the  sealing  up  of  the  strata  in  which  petroleum 
is  now  found,  but  to  'a  direct  effect  of  different  pressures. 

Numerous  other  instances  could  be  given,  but  these  are 
sufficient  to  suggest  a  field  of  enquiry  which  might  be  followed 
up  by  laboratory  experiments,  the  results  of  which  might  throw 
light  upon  the  conditions  governing  the  formation  of  mineral 
oils  of  every  grade  and  nature. 

Temperature. — The  evidence  as  regards  the  temperatures  at 
which  petroleum  may  be  formed  in  nature  is  no  less  interesting. 
It  is  evident  that  if  depth-temperature  alone  is  to  be  considered, 
and  in  the  case  of  most  oilfields  it  is  impossible  to  postulate 
any  other  phenomenon  capable  of  causing  a  rise  in  temperature, 
there  is  no  very  great  range  of  temperature  available.  In  the 
case  of  Point  Ligoure  a  rise  in  temperature  of  40  degrees 
Fahrenheit  would  be  all  that  could  be  granted,  In  the  case 
of  the  Yaw  Valley  it  would  not  be  safe  to  calculate  upon  a  rise 
in  temperature  of  more  than  52  degrees  or  53  degrees. 

Thus  we  see  that  the  researches  upon  peat  furnish  an  in- 
teresting and  attractive  suggestion  as  to  the  conditions  under 
which  mineral  oils  are  formed  in  nature.  High  pressure  and 
comparatively  low  temperature  are  the  conditions  under  which 
petroleum  can  be  produced  from  the  vegetable  matter  of  peat 
masses,  and  similar  conditions  are  at  the  least  easily  obtained 
in  the  strata  of  what  are  now  oilfields.  The  high  temperatures 
required  for  the  destructive  distillation  of  animal  fats  to  form 
distillates  consisting  of  a  mixture  of  hydrocarbons  similar  to 
natural  petroleum,  are  not  only  unnecessary,  but  can  hardly  be 
assumed  to  be  within  the  range  of  possibility. 

Salt  and  Brine. — One  other  interesting  and  even  puzzling 
feature  about  many  oilfields  is  the  frequent  association  of 
petroleum  with  brine  or  rock-salt. 

The  first  oilwell  drilled  in  America  was  intended  to  reach 
brine  and  not  petroleum,  and  in  many  other  countries  it  has 
been  in  the  search  for  brine  or  salt  that  oil  has  been  found.  In 
very  many  oilfields,  also,  the  water  associated  with  the  petro- 
leum or  occurring  in  porous  beds  below  it,  and  also  frequently 
above  it,  is  brackish  or  even  highly  impregnated  with  sodium 
chloride.  In  mud- volcanoes  also,  the  water  and  mud  discharged 
are  almost  invariably  saline. 


PROCESSES   OF   FORMATION  33 

It  has  been  claimed  that  the  occurrence  of  this  brine  is 
confirmatory,  in  some  unexplained  manner,  of  the  theory  that 
it  is  in  marine  strata  and  from  marine  organisms  that  petroleum 
has  been  formed,  and  the  well-known  antiseptic  properties  of 
common  salt,  under  subaerial  conditions,  be  it  noted,  have  even 
been  adduced  as  being  likely  to  favour  the  partial  and  selective 
decomposition  of  animal  matters  which  would  be  necessary  if 
petroleum  is  to  be  formed  from  them. 

Into  this  speculation  the  author  does  not  care  to  venture, 
for  lack  of  sufficient  detailed  evidence.  But  it  must  be  admitted 
that  the  terrestrial  vegetation  theory  does  not  on  the  face  of  it 
explain  the  presence  of  these  saline  waters,  nor  does  their  origin 
from  vegetable  matter  seem  possible. 

Without  attempting  an  explanation,  however,  it  is  possible 
to  review  such  facts  as  bear  upon  the  problem  and  to  consider 
how  far  these  facts  may  indicate  a  possible  solution. 

In  the  first  place  it  is  necessary  to  ascertain  whether  brine 
and  petroleum  are  always  associated  or  not;  in  other  words 
whether  the  former  is  an  essential  concomitant,  or  whether  its 
occurrence  may  or  may  not  be  due  to  causes  not  in  themselves 
directly  necessary  to  the  formation  of  mineral  oil.  Unfor- 
tunately we  are  at  present  unable  to  answer  this  question  with 
certainty.  In  some  oilfields  a  strong  brine  underlies  or  accom- 
panies the  oil  in  every  petroliferous  band,  in  most  cases  what 
water  is  found  is  slightly  saline  or  brackish,  in  a  few  cases 
there  is  little  evidence  of  salinity.  In  the  famous  Yenangyoung 
field  of  Burma  the  waters  met  with  in  the  upper  oilsands,  or 
in  water-sands  between  them,  are  fresh  or  only  moderately 
brackish,  while  a  distinct  brine  has  been  struck  in  the  lowest 
sand  penetrated  up  to  February,  1911.  In  this  case,  however, 
it  may  be  that  the  upper  waters  have  been  briny  and  have 
been  diluted  by  the  incursion  of  surface  water.  Thus  the 
percolation  downwards  of  fresh  water  may  result  in  the 
occurrence  of  a  small  quantity  of  brine  in  the  oilrocks  being 
overlooked. 

Many  oilfields  contain  regular  beds  of  rock-salt,  e.g.  Luristan, 
Persia  and  Texas,  and  these  deposits  may  be  found  both 
above  and  below  oil-bearing  strata.  Again,  in  Persia  brine 
springs  giving  rise  to  saline  rivers  rise  from  some  of  the  strata 
which  are  approximately  on  the  same  horizon  as  oil-bearing 
rocks  in  neighbouring  districts.  In  the  cases  where  brine  is 

D 


34  OIL-FINDING 

most  conspicuous,  a  suggestive  subject  for  enquiry  is  the  in- 
vestigation of  the  evidence  as  to  the  conditions  under  which 
the  strata  now  containing  brine  have  been  deposited,  while  it 
is  also  necessary  to  take  into  account  the  present  climatic 
conditions  under  which  the  strata  are  observed. 

In  Persia,  in  the  oilfields  of  Luristan,  and  more  especially 
in  the  strata  overlying  the  known  oilrocks,  we  have  almost 
every  possible  proof  of  a  former  desiccation  during  formation 
Ked-coated  inudstones  and  sandstones,  deposits  of  gypsum  on  a 
gigantic  scale,  Brockram-like  breccias  on  the  flanks  of  limestone 
outcrops  unconformably  overlaid,  are  the  rule  throughout  a 
vast  thickness;  of  strata.  Furthermore  there  is  indisputable 
evidence  of  a  contemporaneous  earth-movement  that  shut  off 
basins  and  allowed  the  desiccation  to  take  place.  The  occur- 
rence of  beds  of  rock-salt,  therefore,  can  readily  be  understood, 
quite  apart  from  any  suggestion  of  its  being  essentially  associated 
with  petroleum.  Furthermore,  the  climate  of  this  region 
(Plate  IV)  is  very  dry,  absolutely  rainless  throughout  a  great 
part  of  the  year,  so  that  there  is  no  excess  of  surface  waters  to 
dilute  arid  disguise  the  presence  of  brine  in  the  strata.  The 
importance  of  this  point  concerning  climatic  conditions  at  the 
present  day  can  be  appreciated  when  the  logs  of  the  wells 
drilled  in  the  Maidan-i-Naphtun  field  in  Persia  are  studied. 
Hardly  any  water  has  been  encountered  at  any  depth  in  any 
of  the  wells.  The  significance  of  this  point  will  appear 
shortly. 

In  Baluchistan  in  the  Khatan  oilfield,  a  region  almost 
rainless,  the  waters  associated  with  and  accompanying  the  oil 
are  impregnated  with  salts,  but  instead  of  sodium-chloride  it  is 
largely  the  sulphates  of  sodium  and  calcium  that  are  present. 
These  salts  occur  frequently  throughout  great  belts  of  the  dry 
zone,  and  are  characteristic  generally  of  arid  regions,  quite  apart 
from  oilfields.  Such  evidence  suggests  that  there  may  not  be 
any  essential  connection  between  the  occurrence  of  salt  or  brine 
and  petroleum. 

The  whole  question,  however,  requires  exhaustive  research 
before  it  can  be  decided  whether  or  no  the  oil  and  brine  are 
due  to  the  same  chemical  action,  whether  they  are  different 
effects  of  the  same  causes,  or  .whether  their  association  is  merely 
adventitious.  In  the  answers  to  these  questions  probably  lies 
one  of  the  most  illuminating  generalizations  yet  to  be  made  in 


PROCESSES    OF   FORMATION  35 

the  geological  study  of  petroleum,  and  one  which  may  be  of 
great  practical  value  to  those  who  have  to  exploit  new 
oilfields. 

What  is  required  is  a  large  number  of  analyses  of  the 
brines  and  brackish  waters  found  accompanying  or  underlying 
the  petroleum  in  an  oilrock  or  discharged  from  a  mud- volcano. 
In  each  case  it  must  be  known  from  what  depth  the  water  was 
obtained,  with  what  particular  kind  of  oil  it  was  associated, 
paraffin  or  asphaltic,  high  or  low  grade,  whether  sulphur 
compounds  were  present  in  the  oil,  and  if  so,  in  what  percentage, 
and  whether  there  has  been  any  possibility  of  surface  waters 
having  percolated  downwards  and  mingled  with  the  brine  or 
brackish  water.  Without  precise  data  of  this  kind  it  is 
dangerous  to  generalize. 

The  only  suggestion  that  the  author  would  put  forward  is 
that  it  must  not  be  forgotten  that  salt  and  petroleum  may  be 
entirely   unconnected.      Every  sedimentary   rock — and   many 
igneous  rocks  for  that  matter — contains  either  sodium  chloride 
or  ingredients  which  could  furnish  that  salt  if  the  rock  were 
sufficiently  lixiviated.     Where  water  is  in  excess,  as  in  water- 
bearing strata,  the  percentage  of  sodium  chloride  is  so  small  as 
to  be  inappreciable,  but  where  water  is  in  smaller  quantity  and 
has  percolated  through  a  considerable  thickness  of  strata  it  is 
possible  that  a  considerable  concentration  of  saline  matter  in 
solution  may  have  taken  place.     Now  we  have  seen  that  one 
of  the  probable  conditions  under  which  petroleum  has  been 
formed  is  the  presence  of  a  limited  quantity  of  water.     Much 
of  the  hydrogen  also  may  be  utilized  in  the  formation  of  the 
mixture  of  hydrocarbons  which  we  know  as  crude  petroleum, 
but  this  is  very  doubtful,  as  it  would  necessarily  involve  the 
oxidation  of  any  oxidizable  material  in  the  vicinity.     Hpwever 
this   may  be,  it   is   evident   that    any  residual   water   might 
become  a  fairly  concentrated  solution  of  saline  matter.     As  we 
have  seen  that  petroleum  is  formed  in  what  we  may  consider  a 
closed  retort,  circulation  of  subterranean  waters  and  percolation 
of  water  from  upper  strata  might  be  impossible  or  only  possible 
to  a  very  slight  extent,  and  a  brine  associated  with  the  oil  or 
underlying  it  might  survive  without  dilution  till  the  oil-bearing 
strata  are  pierced  by  the  drill.      The  evidence  of  desiccation 
In  the  strata  overlying  petroliferous  rocks  in  many  oilfields 
shows   that  excess  of  water  is  not  a  probable  condition    in 


36  OIL-FINDING 

the  series  containing  oil,  for  where  rainfall  is  scanty  and 
evaporation  rapid  the  absorption  of  water  by  the  strata  must 
be  minimised. 

This  hypothesis  as  to  the  reason  why  saline  water  is  usually 
found  in  association  with  petroleum  is  only  put  forward  as  a 
suggestion,  which  must  be  tested  by  application  to  facts  as 
observed ;  it  is  merely  stated  now  as  a  guide  to  the  direction  in 
which  future  research  may  prove  profitable. 

There  is  one  other  point  in  connection  with  the  formation 
of  petroleum  which  cannot  be  too  clearly  insisted  upon.  It  is 
the  common  practice  to  distinguish  between  oils  of  asphaltic 
base  and  oils  of  paraffin  base,  and  they  are  often  spoken  and 
written  about  as  if  they  were  entirely  different  minerals.  In 
some  cases  it  has  even  been  suggested  that  they  have  been 
formed  from  different  raw  materials. 

But  there  is  actually  no  hard  and  fast  line  between  asphaltic 
and  paraffin  oil;  many  asphaltic  oils  contain  a  percentage  of 
solid  paraffin,  and  many  so-called  paraffin  oils  can  be  made  by 
careful  distillation  to  yield  a  residue  of  asphalt.  In  fact,  there 
is  less  difference  between  different  crude  petroleums  than 
between  different  coals,  which,  as  is  well  known,  show  every 
gradation  from  the  least  mineralized  lignite  with  a  high  per- 
centage of  water,  through  bituminous  coals  and  gas-coals  to 
anthracite,  and,  perhaps,  finally  even  to  graphite. 

It  has  been  shown  that  the  light  paraffin  oils  of  Burma,  with 
percentages  of  solid  paraffin  up  to  as  much  as  thirteen,  and  the 
heavy  asphaltic  oils  of  Trinidad  can  both  be  proved  to  have 
been  formed  from  vegetable  matter,  while  the  paraffin  oils  of 
Trinidad,  with  percentages  of  solid  paraffin  up  to  six  (though 
they  occur  under  slightly  different  conditions  from  those 
in  which  the  asphaltic  oils  are  found,  in  the  former  case 
impregnating  thin  oilsands  very  well  sealed  up  amidst 
thick  masses  of  clay),  give  no  evidence  of  an  essentially 
different  origin. 

To  account  for  the  differences  in  grade  and  class  of  crude 
petroleum,  we  must  look  to  variations  in  the  conditions  of 
formation ;  different  pressures  are  probably  the  most  important 
factors,  but  differences  in  temperature,  relative  quantity  of 
water  present,  and  many  other  local  conditions  probably  all 
play  their  parts.  In  these  questions  there  is  need  for  much 
research  and  experimental  work  in  the  laboratory,  and  it  is 


PROCESSES   OF   FORMATION  37 

hardly  within  the  province  of  the  geologist  to  speculate  upon 
the  effects  of  the  environment  to  which  the  raw  material  was 
subjected.  It  is,  however,  the  geologist's  task  to  deduce  and 
discover  as  far  as  possible  what  that  environment  must  have 
been,  so  that  armed  with  the  knowledge  thus  gained  the 
chemist's  task  may  be  simplified. 


CHAPTER   III 

THE  MIGRATION,   FILTRATION,    AND    SUBTER- 
RANEAN STORAGE   OF   PETROLEUM 

IT  is  necessary  now  to  consider  what  may  happen  to  the  crude 
petroleum  after  it  has  heen  formed,  what  movements  are 
possible  for  it,  and  the  reasons  for  those  movements,  how  it  is 
concentrated  and  stored,  and  how  it  may  be  affected  in  grade 
or  quality  by  the  conditions  to  which  it  is  subjected.  The 
migration,  filtration,  and  storage  of  oil  in  nature  are  subjects 
so  inextricably  connected  that  they  can  hardly  be  considered 
apart ;  they  must  all  be  understood  by  the  geologist  if  he  is  to 
be  capable  of  reading  field  evidence  correctly  and  assigning  its 
true  significance  to  every  indication  which  he  may  have  to 
consider  of  the  presence  of  petroleum,  at  the  surface  or  in 
a  well. 

The  causes  for  the  migration  of  oil  are  earth-movement, 
hydrostatic  pressure,  and  gas  pressure.  There  are  many  factors 
which  determine  movements  of  oil,  but  directly  or  indirectly 
all  movements  are  due  to  these  three  causes.  The  theory  that 
oil  is  underlain  by  water  or  brine  and  has  been  floated  up  by 
the  heavier  liquid  through  porous  strata,  and  thus  by  the 
hydrostatic  pressure  of  the  water  forced  towards  the  crests  of 
flexures  or  to  outcrop,  is  pretty  generally  accepted,  and  certainly 
in  fields  such  as  those  of  the  Eastern  States  in  America,  where 
the  strata  often  lie  at  low  angles  over  great  stretches  of  country 
with  very  small  and  gentle  flexures  and  disturbances,  and  the 
porosity  of  the  rocks  does  not  vary  sufficiently  to  hinder 
migration,  there  may  have  been  a  great  lateral  progression  of 
petroleum  towards  the  localities  best  adapted  for  storing  it. 
But  cases  are  not  always  so  simple,  and  to  assume  that  in  any 
oilfield  the  petroleum  contents  have  originated  at  a  great 
distance,  and  have  only  reached  their  present  position  after  a 
wearisome  journey,  is  quite  another  matter.  The  insistence 

38 


THE   MIGRATION    OF   PETROLEUM  39 

upon  the  migratory  feats  of  petroleum  has  arisen  to  some  extent, 
at  least,  from  the  desire  to  account  for  the  formation  of  the 
hydrocarbons  from  animal  matter.  Thus,  on  the  theory  that 
the  oil  of  the  Californian  and  Texas- Louisiana  fields  has  been 
formed  from  the  soft  parts  of  foraminifera  preserved  in  thick 
masses  of  shales  and  clays,  it  is  necessary  to  postulate  a  migra- 
tion of  each  minute  particle  through  almost  impervious  strata 
in  a  certain  direction  to  form  an  accumulation  in  a  porous 
stratum.  To  attribute  such  a  movement  to  the  hydrostatic 
pressure  of  water  is  perhaps  to  attach  too  great  importance  to 
an  action  which  in  porous  and  inclined  strata  does  without 
doubt  take  place.  But  it  has  already  been  shown  on  what  very 
doubtful  evidence  a  foraminiferal- origin  theory  rests.  If  on 
the  contrary  the  oil  is  formed  from  accumulations  of  vegetable 
matter,  it  is  not  necessary  to  postulate  extensive  migration  as 
a  rule ;  strata  capable  of  containing  the  petroleum  are  usually 
at  hand,  and  in  these  strata  it  will  be  found.  The  Tertiary 
Series  in  Burma  and  Trinidad,  where  great  thicknesses  of  strata 
of  estuarine  origin  are  present,  supply  abundance  of  evidence 
on  this  point,  while  lignitic  or  carbonaceous  beds  contempora- 
neous with  the  oil-bearing  strata,  and  at  no  great  distance  from 
them  give  evidence  of  the  presence  of  the  raw  material,  and 
suggest  that  no  great  or  extensive  migration  is  necessary. 

Hydrostatic  Pressure. — It  is  the  geological  structure  and 
the  porosity  of  the  oilrocks  that  determine  the  effects  of 
hydrostatic  pressure.  The  rocks  must  be  sufficiently  porous 
to  admit  of  free,  if  slow,  movements  of  the  aqueous  contents, 
and  the  strata  must  be  sufficiently  inclined  to  determine  the 
direction  of  movement.  Thus  towards  the  crests  of  anticlines, 
both  laterally  and  upwards,  there  must  in  nearly  every  case 
be  a  gradual  migration  of  oil  by  the  gradual  replacement  by 
water  in  the  lower  levels,  when  there  is  a  sufficient  difference 
in  the  specific  gravities  of  the  liquids.  In  a  subsequent 
chapter  the  various  structures  that  favour  such  migration  will 
be  dealt  with. 

The  question  of  specific  gravity  becomes  in  some  fields  a 
matter  of  great  importance.  The  fact  seems  to  have  been  lost 
sight  of  occasionally  that  a  heavy  asphaltic  of  say  0*95  specific 
gravity  or  higher  will  be  affected  much  more  slowly  than  a 
light  oil  of  0*72  specific  gravity.  Consequently  in  considering 
lateral  or  upward  movements  of  petroleum  the  particular 


40  OIL-FINDING 

grade  of  the  petroleum  must  be  taken  into  account.  To  over- 
come the  friction  and  the  viscosity  of  the  oil  which  must 
necessarily  retard  percolation,  a  considerable  advantage  in 
specific  gravity  must  be  possessed  by  the  water.  Thus  to 
generalize  on  the  subject  of  migration  of  oil  from  facts  ascer- 
tained in  the  Pennsylvania  fields,  where  a  light  paraffin  oil  is 
found,  and  to  apply  the  generalizations  to  such  fields  as 
those  of  California,  or  even  Baku,  where  an  asphaltic  oil  of 
heavier  gravity  is  the  rule,  is,  to  say  the  least,  very  unsafe. 

Gas  Pressure. — Another  cause  of  what  may  properly  be 
called  migration  of  oil  is  gas  pressure.  The  gas  may  not 
exist  as  such  in  the  strata,  being  dissolved .  and  occluded  to  a 
great  extent  in  the  petroleum,  or  the  pressure  may  be  too 
great  to  allow  of  the  existence  of  gas  if  it  is  below  the 
"  critical  temperature."  In  that  case  the  gas  will  be  in  a 
potentially  gaseous  state,  and  must  exert  an  enormous 
pressure  in  seeking  to  find  space  in  which  to  expand  to  the 
gaseous  state.  The  terrific  force  with  which  such  gas  is  dis- 
engaged on  the  striking  of  a  prolific  well  is  sufficient  evidence 
on  this  point,  as  it  is  now  admitted  that  gas  pressure  is  the 
chief  if  not  the  sole  cause  of  fountains  or  flowing  oilwells. 
This  gas,  dissolved,  occluded  in  or  mechanically  associated  with 
the  oil  must  exercise  pressure  in  all  directions,  and  here  again 
comes  a  point  that  has  frequently  been  lost  sight  of.  It  is 
often  assumed  that  the  movements  of  gas  and  oil  must  be 
directly  or  indirectly  upward,  and  this  has  often  caused 
deplorable  errors  to  be  made  in  the  location  of  oilwells  and 
in  the  deepening  of  wells  long  after  they  have  passed  through 
the  lowest  strata  in  which  there  is  any  hope  of  oil  being  struck. 
A  "  show  "  of  gas  in  a  well  has  only  too  frequently  been 
understood  as  a  sign  that  oil  must  lie  beneath. 

But  if  gas  exerts  pressure  in  all  directions  it  will  migrate 
in  all  directions  till  stopped  by  some  impervious  stratum. 
Thus  both  laterally  and  downwards  there  may  be  a  migration 
of  gas  carrying  with  it  probably  small  quantities  of  the  lighter 
constituents  of  the  oil.  The  oil  will  gradually  be  trapped 
during  the  migration,  especially  by  argillaceous  strata,  so  that 
the  gas  finally  reaches  furthest  from  the  parent  source.  It  is 
owing  to  this  that  we  find  gasfields  spreading  beyond  the 
confines  of  an  oilfield,  and  profitable  productions  of  gas  may 
be.  obtained  near  a  prolific  oilfield  but  in  localities  where  no 


THE   FILTRATION    OF   PETROLEUM          41 

oil  can  be  struck  and  where  the  strata  may  be  substantially 
waterlogged.     Instances  of  this  are  not  uncommon  in  Burma. 

Again,  gas  may  be  found  beneath  the  oil-bearing  strata, 
and  may  be  evolved  from  clays  and  other  almost  impervious 
rocks  long  after  the  porous  oil-bearing  strata  above  have  been 
removed  by  denudation.  The  impregnation  of  strata  uncon- 
formably  overlaid  by  oil-bearing  rocks  has  been  observed  in 
many  parts  of  the  world ;  good  instances  are  recorded  from 
Alaska,  where  metamorphic  rocks  have  been  impregnated  from 
the  Tertiaries  above  them,  and  from  Galicia,  where  Cretaceous 
strata  contain  oil  derived  from,  an  overlying  Tertiary  Series. 
Such  cases  of  impregnation  have  also  come  under  the  writer's 
personal  observation  in  Baluchistan  and  Trinidad.  In  Burma 
also  there  is  some  evidence  of  deep  wells  passing  through  the 
petroliferous  Pegu  Series  and  striking  oil  in  the  unconformable 
series  beneath.  In  some  of  these  cases  the  strata  beneath  are 
argillaceous,  so  that  they  contain  little  more  than  gas,  and 
perhaps  a  little  filtered  oil,  the  exudation  of  which  when 
exposed  at  the  surface  is  naturally  very  slow. 

In  the  south-eastern  corner  of  Trinidad  slow  evolutions  of 
gas  may  be  seen  from  an  outcrop  of  clay  of  the  Cretaceous 
Series,  which  is  not  petroliferous  in  the  district,  but  which  is 
overlaid  unconformably  in  the  immediate  vicinity  by  oil-bearing 
Tertiary  sands.  In  the  Piparo  district  of  the  same  island  the 
discharge  of  gas  in  one  locality  has  been  sufficient  to  form  two 
small  mud-volcanoes  on  an  outcrop  of  Cretaceous  clay  which 
was  not  originally  petroliferous.  In  this  instance,  however, 
the  volcanoes  may  be  fed  from  some  more  porous  strata 
beneath,  which  have  been  more  completely  impregnated  from 
the  Tertiaries. 

Filtration  Effects. — Any  oil  appearing  with  the  gas  in  such 
cases  will  probably  be  well  filtered  and  to  a  large  extent 
decolorized.  Professor  Clifford  Richardson  has  proved  that 
by  continued  filtration  through  clay  solutions  of  asphalt  and 
petroleum  of  any  kind  may  be  almost  completely  decolorized 
owing  to  the  absorptive  and  "  adsorptive "  properties  of  the 
clay.  The  fraction  "  adsorbed  "  cannot  be  extracted  again  by 
treatment  with  solvents,  and  so  is  distinguished  from  that 
absorbed.  This  phenomenon  is  very  suggestive,  as  similar 
conditions  may  easily  be  reproduced  in  nature.  Where  oil 
is  obtained  from  argillaceous  rocks  it  is  almost  invariably 


42  OIL-FINDING 

light  in  gravity  and  colour,  and  productions  are  not  as  a  rule 
large  nor  gas  pressure  great.  The  water-clear  oil  of  Kala- 
Deribid  in  Persia  (Plate  V)  is  the  most  striking  instance  that 
has  come  within  the  writer's  observation.  This  oil,  which  is 
perfectly  "  water-white,"  collects  very  slowly  in  small  holes 
dug  in  the  outcrop  of  a  fine-grained,  compact  shale,  exposed 
in  a  small  stream  valley.  There  is  very  little  evolution  of 
gas  in  this  case,  only  a  few  bubbles  being  noticed,  as  compared 
with  the  brisk  evolution  so  frequently  observed  from  an  outcrop 
of  oilrock.  The  "  show,"  though  on  the  crest  of  a  large  and 
sharp  asymmetrical  anticline  (Plate  VI),  is  not  concentrated 
towards  the  actual  line  of  crest,  but  distributed  for  some  20 
or  30  yards  through  the  outcrop  of  the  shale  on  the  gently 
dipping  flank  of  the  flexure.  This  surface  indication,  in  fact, 
differs  essentially  from  the  usual  show  of  oil  on  an  anticlinal 
crest ;  the  petroleum  does  not  seem  to  be  forced  up  or  carried 
up  by  gas,  but  collects  particle  by  particle,  just  as  water  collects 
in  an  excavation  in  a  water-bearing  sand.  The  greatest  yield 
is  about  four  kerosine  tins  per  day. 

Close  above  the  shales  occur  several  outcrops  of  rather 
loosely  compacted  sandstone,  which  have  all  the  appearance  of 
weathered  oilsands,  but  which,  beyond  traces  of  sulphur, 
contain  no  sign  of  oil.  Such  traces  of  sulphur  are  often  the 
last  surviving  evidence  (in  a  thoroughly  lixiviated  sand)  of  the 
former  presence  of  oil  which  contained  sulphur. 

The  author's  theory  with  regard  to  this  water-clear  oil  is 
that  it  is  a  filtered  residue  yielded  slowly  by  the  almost 
impervious  argillaceous  rock,  that  we  must  look  for  its  origin 
in  oilsands  lying  above  the  shale,  and  that  it  affords  an  instance 
of  downward  migration  of  oil,  only  the  filtered  remains  of  which 
have  been  preserved  by  the  less  easily  weathered  shale. 

Filtered  oils,  varying  in  colour  from  water- clear  to  that  of 
a  well-matured  brandy,  which  are  obtained  in  small  but  payable 
quantities  from  shallow  wells  in  the  limestone  of  Eamri  Island 
off  the  coast  of  Arakan,  have  probably  a  similar  origin.  The 
yield  is  steady  and  slow,  the  gas  pressure  small,  while  inspis- 
sation  has,  as  in  the  case  of  Kala-Deribid,  removed  most  of  the 
more  inflammable  fractions,  giving  the  oil  a  high  flash-point, 
and  enabling  it  to  be  burnt  in  ordinary  lamps  without  distil- 
lation. In  this  case  also  the  overlying  series  is  petroliferous, 
and  otlshows  on  a  large  scale  with  explosive  discharge  of 


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THE    FILTRATION    OF   PETROLEUM          43 

gas    from    younger    strata    are     not     far    distant,    e.g.    Faule 
Island. 

In  Baluchistan  a  somewhat  different  case  of  migration  into 
older  strata  may  be  observed.  The  impregnation  is  only  along 
joint  planes  and  in  beds  of  slightly  greater  porosity  in  a 
compact  limestone,  while  the  oil  is  a  dark  heavy  residue 
containing  sulphur  and  very  little  light  oil.  The  shows  occur 
on  the  flanks  of  a  range  of  hills  formed  of  limestone,  anticlinal 
in  structure,  and  overlaid  by  a  thick  series  of  shales.  It  is 
only  at  the  edge  of  or  beneath  the  outcrop  of  the  shale  that  any 
appreciable  production  of  oil  has  ever  been  obtained  (e.g. 
Khatan),  and  the  oil  is  very  heavy,  contains  a  large  proportion 
of  sulphur  compounds,  and  is  accompanied  by  warm  sulphur 
springs.  It  seems  perfectly  clear  that  we  are  dealing  with  the 
inspissated  residue  of  a  partial  impregnation  which  took  place 
before  denudation  had  laid  bare  the  series  so  deeply,  and  that 
now  only  the  ail-but  final  results  of  inspissation  are  in  evidence 
to  indicate  that  impregnation  of  lower  strata  has  taken  place. 
The  overlying  shales  are  in  places  distinctly  bituminous,  and  it 
is  from  their  outcrop  not  many  miles  distant  that  a  bituminous 
coal  rich  in  pyrites  is  mined. 

A  more  striking  instance  of  much  the  same  phenomenon, 
and  one  that  can  be  more  easily  and  completely  studied,  is 
afforded  by  San  Fernando  Hill  in  Trinidad.  The  hill  is  formed 
of  an  inlier  of  a  peculiar  rock  called  "argiline"  by  Messrs. 
Wall  and  Sawkins,  and  it  forms  the  core  of  an  anticline  in  the 
petroliferous  Tertiaries  which  overlie  it.  The  argiline,  though 
an  exceedingly  fine-grained  rock,  has  been  impregnated  through- 
out, and  owing  to  its  homogeneous  nature  and  closeness  of 
grain  has  been  enabled  to  retain  the  impregnation  under 
weathering  influences  for  a  considerable  time.  In  the  numerous 
quarries  opened  in  this  argiline,  a  crust  usually  some  six  to 
eight  feet  thick  of  the  weathered  material  is  observed,  separated 
sharply  from  the  part  still  impregnated ;  the  line  between 
weathered  and  un weathered  argiline  crosses  the  bedding  obliquely 
in  many  places.  At  the  north-eastern  end  of  the  hill  sticky 
inspissated  oil  has  exuded  in  considerable  quantity,  so  much  so 
that  a  syndicate  was  once  formed  to  work  it,  but  after  doing 
some  excavation  the  enterprise  was  abandoned  as  unprofitable. 
Similar  attempts  are  often  made  to  obtain  oil  where  some  such 
deceptive  "  show  "  has  tempted  men  of  enterprise,  but  without 


44  OIL-FINDING 

geological  knowledge,  to  commence  development  work,  arid  it 
is  largely  from  such  unsuccessful  attempts  that  the  popular 
idea  of  the  great  uncertainty  of  oil  exploitation  has  arisen. 

Another  instructive  example  is  furnished  by  the  first  well 
drilled  by  a  company  now  operating  in  Trinidad.  The  well  was 
commenced  below  the  horizon  of  the  oil-bearing  rocks  of  the 
district,  and,  after  passing  at  shallow  depth  through  strata  with 
slight  indications  of  oil,  entered  a  thick  series  of  clay  from, 
which  a  certain  quantity  of  gas  issued.  This  gas  assisted  to 
puddle  the  clay,  which  caved  badly,  and  made  it  rise  in  the 
bore-hole  and  thus  cause  great  difficulty  in  the  drilling.  The 
clays  at  this  horizon  are  of  great  thickness  and  only  contain 
small,  inconstant,  and  insignificant  beds  of  oilsand.  After 
struggling  for  months  with  these  argillaceous  strata  and  the 
gas,  the  well  was  abandoned,  having  reached  a  depth  of  only 
some  500  feet.  It  was  probably  the  occurrence  of  gas  that 
induced  the  company  to  persevere  with  the  drilling,  although 
they  had  been  warned  before  the  derrick  was  erected  that  the 
geological  sequence  of  strata  had  been  worked  out  carefully,  and 
that  the  well  would  certainly  prove  a  failure. 

These  instances  are  merely  quoted  to  show  of  what  practical 
importance  it  is  that  the  probability  of  downward  migration  of 
petroleum  and  gas,  even  into  almost  impervious  strata,  should 
be  recognized. 

Another  theory  that  is  sometimes  expressed  regarding  migra- 
tion of  oil  is  that  it  has  been  present  in  some  particular  area, 
but  has  escaped,  by  means  of  faults  in  the  strata,  and  so  is  no 
longer  available  nor  can  be  struck  in  a  well.  This  is  one  of 
the  many  suggestions  made  about  faults  by  those  whose  personal 
or  practical  acquaintance  with  geological  work  is  small,  but 
who  make  use  of  the  idea  of  faulting  as  a  sort  of  deus  ex 
machina  to  account  for  something  which  they  do  not  under- 
stand or  have  been  unable  to  explain.  It  is  reminiscent  of  an 
antique  method  in  geological  mapping,  the  observer  when 
involved  in  serious  difficulties  boldly  mapping  a  theoretical 
fault  and  starting  afresh.  In  books  on  the  subject  of  petro- 
leum, when  faults  are  mentioned  the  word  is  usually  followed 
by  the  words  "  fissures "  and  "  crevices."  "  Crevice,"  by  the 
way,  is  a  favourite  word  with  the  careless  driller  who  has 
provided  himself  with  a  "  fishing-job,"  and  who  lays  the  blame 
for  the  disaster  upon  a  "  crevice,"  which,  suddenly  entered 


THE    FILTRATION    OF   PETROLEUM          45 

upon,  caused  too  great  a  strain  to  be  put  upon  some  part  of 
his  string  of  tools  or  cable. 

Faults,  fissures,  and  crevices  are  stated  to  have  considerable 
effects  upon  the  underground  storage  of  petroleum  by  affording 
channels  which  allow  the  oil  to  escape  upwards,  downwards,  or 
laterally,  and  to  have  disappeared  from  the  rock  in  which  it 
was  stored. 

Let  it  be  admitted  at  once  that  faults  do  not  infrequently 
affect  oilfields  either  favourably  or  unfavourably,  and  have  often 
a  notable  local  effect  in  increasing  production.  Their  effects 
are  purely  structural,  and  will  be  dealt  with  in  a  subsequent 
chapter  on  geological  structure.  As  channels  for  migration  of 
oil  to  any  important  extent  they  do  not  act,  for  the  simple 
reason  that  a  "fault- fissure"  as  the  term  is  used  in  geological 
parlance  is  not  an  open  fissure  in  the  ordinary  sense  of  the 
word.  Open  fissures  are  in  any  case  very  rare  in  nature,  and 
only  occur  in  limestone  formations  or  in  hard  igneous  or  meta- 
morphic  rocks,  and  then  usually  comparatively  near  the  surface. 
Were  any  open  fissure  to  be  formed  in  soft  Tertiary  strata,  the 
pressure  would  be  sufficient  to  close  it  very  rapidly,  while  if 
petroleum  were  to  commence  migrating  by  such  a  channel  the 
fissure  would  soon  be  clogged  by  inspissating  oil  and  the  sand 
or  clay  brought  with  it.  The  storage  of  petroleum  in  any 
locality  necessitates  a  more  or  less  impervious  cover,  usually 
of  considerable  thickness,  and  this  covering  would  require  to 
be  completely  dislocated,  a  fissure  opened  and  prevented  from 
becoming  sealed  before  there  could  be  any  possibility  of  the 
escape  of  petroleum  in  quantity.  Where  the  covering  is  largely 
of  soft  argillaceous  strata  such  a  phenomenon  is  manifestly 
impossible.  Another  point  also  falls  to  be  considered;  even 
with  an  oil-bearing  series  entirely  exposed  at  outcrop,  the 
petroleum  contents  are  dissipated  by  exudation  at  the  surface 
very  slowly.  At  the  depth  of  some  hundred  or  even  thousands 
of  feet  such  action  into  a  narrow  open  fissure  could  not  but 
be  very  gradual. 

Where  one  does  obtain  evidence  of  a  form  of  migration  to 
which  the  expression  "  intrusion "  may  be  applied,  is  in  veins 
of  manjak,  which  term  is  used  to  include  gilsonite  and  its  con- 
geners, and  ozokerite.  Manjak  is  to  an  asphaltic  oil  what 
ozokerite  is  to  one  of  paraffin  base.  They  are  inspissated  oil 
in  veins  which  have  actually  been  intruded  usually  in  a  vertical 


46  OIL-FINDING 

or  highly  inclined  position  from  oil-bearing  strata  below,  and 
the  material  has  consolidated  without  reaching  the  surface. 
Occasionally  such  veins  may  be  found  along  lines  of  fault,  but 
all  those  with  which  the  writer  is  familiar  are  either  along 
bedding-planes  or  along  minor  slip-planes  and  joints  in  thick 
masses  or  argillaceous  strata.  The  phenomena  associated  with 
manjak  veins  will  be  dealt  with  more  fully  later ;  the  point  to 
be  noted  at  present  is  that  if  petroleum  did  migrate  to  any 
extent  along  fissures,  cracks,  or  fault-planes,  we  should  find 
abundant  evidence  of  its  having  done  so  in  veins  of  manjak 
or  ozokerite.  But  these  phenomena,  though  known  in  many 
parts  of  the  world,  cannot  be  said  to  be  common  occurrences 
in  oilfields,  while  faults  are  frequent  to  a  greater  or  less  extent 
everywhere  that  earth-movement  has  been  in  operation,  and 
there  is  hardly  an  oilfield  that  is  without  some  evidence  of 
faulting. 

From  all  these  considerations  it  will  be  seen  that  the 
migration  of  petroleum  is  a  very  circumscribed  action,  and 
cannot  be  called  upon  to  explain  any  very  widespread 
phenomena  in  oilfields.  To  put  it  briefly,  petroleum  goes 
where  it  can,  but  from  the  very  nature  of  the  conditions  under 
which  it  has  been  formed  and  under  which  it  is  preserved 
its  migrating  movements  are  checked  and  hindered  in  almost 
all  directions.  Thus  when  earth-oils  are  discovered  in  any 
locality,  we  are  almost  justified  in  applying  to  them  the  famous 
conclusions  of  the  gentleman  who  devoted  his  life  to  research 
upon  the  subject  of  the  "  fiery  flying  serpents  in  the  wilderness," 
with  special  attention  to  their  origin  and  subsequent  history : 
(1)  "  They  was  there  all  the  time,"  and  (2)  "  they  stayed  where 
they  was." 

Subterranean  Storage. — The  relative  porosity  of  strata  is  one 
of  the  determining  factors  in  the  movements  of  oil,  and  the 
selection  of  a  reservoir  rock.  Oil  will  find  the  nearest  available 
porous  strata  and  will  impregnate  them.  Given  sufficient  time 
and  pressure  it  will  impregnate,  and  even  to  some  extent  force, 
its  way  through,  an  apparently  impervious  clay,  but  it  will 
select  the  most  porous  stratum  to  impregnate.  This  is  the 
reason  that  a  gas-show,  with  a  slight  show  of  light  and  perhaps 
light-coloured  oil,  is  so  often  struck  in  a  well  some  little 
distance  above  the  main  oilrock.  It  is  a  filtered  oil  which  has 
gradually  accumulated  in  a  porous  band,  after  passing  through 


SUBTERRANEAN    STORAGE  47 

the  almost  impervious  cover  of  the  true  oil-bearing  stratum. 
In  most  cases,  however,  it  is  only  gas  that  is  found  under  these 
conditions. 

The  great  majority  of  oil-bearing  rocks  are  arenaceous,  sand- 
stones of  all  kinds,  grits  or  conglomerates,  but  some  of  the 
world's  most  famous  oilrocks  are  limestones  and  dolomites. 
In  the  case  of  calcareous  rocks  it  is  probably  merely  because 
the  limestone  affords  a  porous  reservoir  that  it  is  found 
impregnated  with  oil,  just  as  in  a  manjak  mine  a  nodule  or 
nodular  band  of  ferrous  and  lime  carbonate,  being  slightly 
more  porous  than  the  surrounding  clay,  will  contain  more 
evidence  of  petroleum  than  the  country  rock.  However,  as  the 
occurrence  of  oil  in  limestones  has  been  made  use  of  as  an 
argument  in  favour  of  the  animal-origin  of  petroleum,  it  is 
necessary  to  examine  the  evidence  carefully.  The  famous 
Trenton  limestone  of  North  America  is  perhaps  as  good  an 
instance  as  could  be  chosen.  It  contains  barren  areas  and  areas 
of  partial  impregnation,  as  well  as  areas  where  great  productions 
of  petroleum  can  be  obtained,  and  it  has  been  the  subject  of 
much  research.  It  has  been  proved  that  in  the  localities  where 
the  rock  is  most  productive  it  is  cavernous  in  structure,  con- 
taining innumerable  small  cavities  which  are  often  drusy,  and 
which  are  found  full  of  oil.  Analysis  has  shown  that  the 
cavernous  variety  of  the  Trenton  rock  is  dolomitized,  the 
dolomitization  naturally  causing  an  increase  in  specific  gravity, 
which  connotes  a  decrease  in  volume,  and  thus  causes  the 
cavities.  The  rock,  in  fact,  over  wide  areas,  though  sometimes 
only  on  selected  horizons,  has  been  formed  into  what  may  be 
compared  to  a  sponge,  and  the  oil  contents  vary  in  quantity 
directly  with  the  degree  of  dolomitization.  It  is  difficult,  if 
not  impossible,  ta  imagine  any  chemical  action  which  will 
bring  about  the  dolomitization  of  a  limestone  and  at  the  same 
time  produce  petroleum,  and  one  is  forced  to  the  conclusion  that 
the  presence  of  the  oil  is  accidental,  and  that  it  has  occupied 
the  cavernous  limestone  simply  because  the  rock  afforded  room 
for  it.  Much  of  the  impregnation,  by  the  way,  may  be  due 
to  downward  migration.  In  this  connection  we  may  bring 
forward  confirmatory  evidence  from  the  cavernous  limestones 
of  Maidan-i-Naphtun  and  Marmatain  in  Persia.  These  are 
the  oil-bearing  strata  from  which  the  Anglo-Persian  Oil  Company 
is  obtaining  such  remarkable  productions,  and  they  were  first 


48  OIL-FINDING 

studied  in  detail  by  the  writer,  They  are  grey  porous  lime- 
stones, containing  innumerable  small  cavities,  generally  lenti- 
cular in  shape  and  attaining  to  a  diameter  of  as  much  as  one 
inch.  The  cavities  are  frequently  drusy ;  their  presence  makes 
the  rock  in  bulk  exceedingly  porous. 

Careful  mapping  of  the  Maidan-i-Naphtun  field  proved  that 
these  are  not  ordinary  limestones,  but  are  of  detrital  origin ; 
they  vary  from  very  coarse  breccias  consisting  of  large  irregular 
blocks  of  limestone  and  sandstone  in  a  calcareous  matrix  on 
the  one  hand,  to  thin  calcareous  sandstones  and  finally  ordinary 
sandstones  on  the  other.  The  thinning  out  of  these  calcareous 
masses,  becoming  sandier  and  occasionally  more  argillaceous  as 
they  thin  out,  is  beautifully  seen.  Surface  indications  of  oil 
occur  in  these  strata  even  where  they  have  thinned  out  into 
bands  of  sandstone  a  few  feet  thick,  but  the  "shows"  are 
greatest  where  the  different  bands  coalesce  into  thick  masses 
of  calcareous  rock.  The  origin  of  the  limestone  fragments, 
which  are  often  most  irregular,  is  the  Cretaceo-Eocene  limestone 
of  Asmari,  a  very  thick  calcareous  formation  which  is  overlaid 
unconformably  by  the  Tertiary  petroliferous  series. 

These  limestones,  being  of  detrital  origin,  cannot  be  brought 
forward  as  evidence  of  the  animal- origin  of  oil,  as  has  been 
done  in  the  case  of  the  Trenton  rock.  Yet  they  present  the 
same  cavernous  and  drusy  characters.  Analysis  to  determine 
whether  doloinitization  has  taken  place  or  not  has  not  yet  been 
undertaken,  but  the  rock  has  all  the  appearance  of  a  dolomite, 
and  the  writer  has  little  doubt  about  the  matter;  the  drusy 
cavities  certainly  contain  crystals  of  dolomite.  The  conclusion 
is  obvious  :  the  cavernous  rock  has  become  impregnated  with 
oil,  because  it  is  the  most  porous  reservoir  available  amidst  a 
thick  series  of  gypsum,  shale,  and  mudstone  beds. 

At  Jemsah,  on  the  Gulf  of  Suez,  very  similar  strata  contain 
the  oil-bearing  beds,  which  are  again  cavernous  dolomites. 

Spindle  Top  gives  another  instance  of  a  cavernous  limestone 
or  dolomite  containing  petroleum  in  quantity.  In  this  case 
the  impregnation  is  probably  due  to  lateral  migration  aided  by 
earth-movement. 

It  is  perhaps  not  out  of  place  to  mention  here  that  lime- 
stone oils  frequently  exhibit  some  differences  from  sandstone 
oils,  and  though  those  differences  may  not  be  essential,  they  may 
be  of  considerable  practical  importance.  Thus  many  limestone 


SUBTERRANEAN   STORAGE  49 

oils  are  noted  for  the  percentage  of  sulphur  which  they  contain  ; 
their  outcrops  are  often  marked  by  sulphur  springs  and  evolution 
of  hydrogen  sulphide,  while  crystals  of  pure  sulphur  may  be 
found  lining  cavities  in  the  oilrock.  Spindle  Top,  Marmatain, 
and  Maidan-i-Naphtun  in  Persia,  and  Khatan,  Spintangi  and 
Kirta  in  Baluchistan  are  instances.  In  these  cases  there  is 
reason  to  believe  that  the  sulphur  compounds  may  not  be 
entirely  original  in  the  petroleum,  but  may  be  due  to  the 
action  of  the  oil  and  water  on  sulphides  contained  in  the  strata. 
In  oilsands  and  their  associated  clays,  pyrites  and  marcasite 
are  not  uncommon,  but  in  the  limestones  of  the  above  men- 
tioned oilfields  these  minerals  are  apparently  absent.  It  is 
possible  that  the  petroleum  may  have  absorbed  and  incorporated 
sulphur  compounds  encountered  during  its  migration  to  and 
through  the  limestone  which  it  now  occupies.  In  the  cases  of 
Khatan  and  Spintangi  the  shales  where  the  oil  originated  are 
full  of  pyrites  in  the  area  where  the  carbonaceous  phase  is  in 
evidence,  and  the  Harnai  Valley  Coal,  as  the  bituminous  coal 
worked  in  these  shales  is  called,  contains  a  large  quantity  of 
pyrites.  This  is  absent  at  Khatan,  but  sulphur  compounds  are 
present  in  the  oil,  and  sulphurous  springs  appear  every  here 
and  there  from  beneath  the  outcrop  of  the  shales. 

Parallel  evidence  can  be  obtained  within  the  confines  of 
Great  Britain ;  in  the  west  of  England  where  the  Carboni- 
ferous limestone  becomes  slightly  bituminous  in  some  locali- 
ties, the  foetid  odour  of  a  fresh  fracture  gives  unmistakable 
evidence  of  the  presence  of  hydrogen  sulphide. 

It  is  not  suggested  that  sandstone  oils  do  not  contain 
sulphur  compounds,  many  of  them  are  unfortunately  very  rich 
in  this,  in  oil,  undesirable  element,  but  there  seems  to  be  some 
condition  affecting  oils  enclosed  in  limestone  which  makes  it 
possible  to  decompose  any  sulphides  present  and  to  incorporate 
a  percentage  of  the  sulphur  in  the  oil,  which  percentage  naturally 
becomes  more  conspicuous  as  the  sulphur  compounds  are  con- 
centrated by  the  inspissation  of  the  petroleum. 

Further  research  is  necessary  upon  this  point,  but  the 
suggestion  of  the  effect  of  environment  on  the  oil  after  its 
formation  is  made  here  as  it  may  be  of  practical  utility.  The 
same  oil  that  impregnates  a  limestone  in  one  locality,  where 
it  is  associated  with  abundant  evidence  of  the  presence  of 
sulphur,  may  be  found  in  a  locality  not  far  distant  impregnating 

E 


50  OIL-FINDING 

a  sandstone,  and  containing  a  smaller  percentage  of  sulphur 
compounds  and  consequently  being  of  higher  quality  and 
better  value. 

It  is  to  sandstones,  however,  that  we  owe  our  principal 
supplies  of  petroleum,  and  almost  every  variety  of  sandstone 
may  be  found  acting  as  an  oil  reservoir. 

Here  one  of  the  popular  ideas  of  the  driller  may  be  sum- 
marily dealt  with.  In  oilfield  work  one  is  frequently  informed 
that  there  are  oilsands,  gas-sands,  and  water-sands,  and  that  they 
have  essential  and  different  characteristics,  while  some  informants 
will  even  go  so  far  as  to  state  that  they  can  tell  by  examination 
of  a  clean  sample  of  sand  to  which  of  these  three  classes  it  belongs. 
These  are  men  often  of  acute  observation,  and  they  may  be  per- 
fectly right  for  a  particular  field,  or  in  a  particular  locality,  but 
to  generalize,  in  one  or  even  several  oilfields  from  the  evidence, 
and  to  expect  the  generalizations  to  prove  true  of  another  field, 
perhaps  in  a  different  country,  is  notoriously  dangerous.  True, 
in  one  field  the  oil-bearing  horizons  may  be  composed  of  a 
certain  kind  of  sand  of  characteristic  coarseness,  colour,  contour 
of  grain,  and  porosity,  while  gas  or  water  may  be  found  in  the 
same  locality  in  arenaceous  strata  of  different  types,  but  a  sand 
m  ly  ch  an  ge  almost  entirely  in  character  within  the  space  of  a 
few  hundred  yards,  and  yet  remain  none  the  less  an  oil  sand, 
gas-sand,  or  water-sand  as  the  case  may  be.  Proceeding  down 
the  flank  of  an  anticlinal  flexure,  or  down  the  pitch  of  a  dome 
structure,  what  was  the  oilsand  near  the  crest  may  be  found 
destitute  of  oil  and  full  of  water. 

Again,  some  sandstones,  especially  those  with  calcareous 
cement,  may  be  so  compact  as  to  be  hardly  capable  of  con- 
taining appreciable  quantities  of  oil,  but  may  contain  gas.  But 
when  one  considers  the  conditions  under  which  such  sands  have 
been  deposited,  it  becomes  obvious  that  the  calcareous  cement 
may  vary  in  quantity  in  different  localities,  and  the  porosity 
of  the  rock  may  vary  as  much.  Thus  the  same  sandbed  may 
be  rich  in  oil  at  a  comparatively  short  distance  from  where  it 
was  merely  a  gas-sand.  A  low-lying  shore,  such  as  may  be 
seen  on  the  eastern  coast  of  Trinidad,  is  an  object  lesson  in 
arenaceous  deposition.  There  every  gradation  from  a  shell- 
bed,  formed  almost  entirely  of  fragments  of  broken  shells,  to 
a  pure  siliceous  sand,  a  muddy  sand  or  a  sand  containing 
vegetable  matter,  which  will  eventually  be  a  carbonaceous  sand, 


SUBTERRANEAN    STORAGE  51 

may  be  seen  accumulating  under  the  action  of  waves  and 
tides.  Each  variety  will  differ  from  the  others  in  size  and 
contour  of  grain,  chemical  composition  and  porosity,  and  all 
are  being  formed  simultaneously  within  a  distance  of,  perhaps, 
less  than  a  mile  of  coast  line. 

It  cannot  be  too  clearly  stated  and  understood  that  an 
oilsand  is  a  sand  containing  oil,  a  gas-sand  one  containing  gas, 
and  a  water- sand  one  containing  water.  Remove  the  contents, 
and  they  are  no  longer  entitled  to  the  names,  though  they  may 
still  be  mapped  geologically  and  designated  as  the  horizons  of 
such  and  such  oil-,  gas-,  or  water-sands. 

Contour  of  Grain. — On  the  subject  of  the  contour  or  shape 
of  the  grains  in  an  oilsaiid  there  has  been  some  confusion  of 
opinion.  Mr.  A.  Beeby  Thompson  in  his  book  on  the  "  Oil- 
fields of  Russia  "  gives  microphotographs  of  sands  from  Baku 
oilwells,  calling  attention  to  their  fairly  well-rounded  character, 
on  the  strength  of  which  he  suggests  that  they  are  wind-blown. 
On  the  other  hand  Professor  Clifford  Richardson  in  his  book 
on  "  The  Modern  Asphalt  Pavement "  gives  microphotographs 
of  the  sand  extracted  from  the  asphalt  of  Trinidad's  famous 
Pitch  Lake  and  washed,  calling  attention  to  the  sharpness  of 
the  grains,  on  the  strength  of  which  he  suggests  that  the 
silica  has  been  deposited  from  solution.  Now  the  sand  grains 
in  Trinidad  asphalt  from  the  Pitch  Lake  are  derived,  as  is  the 
bitumen,  from  the  La  Brea  oil-bearing  group,  on  the  outcrop 
of  which  that  great  asphalt  deposit  has  been  formed.  Here 
then  are  two  authorities  who  have  examined  the  silica  grains 
from  different  asphaltic  oilsands,  one  calling  special  attention 
to  the  roundness  of  grain,  and  the  other  to  the  sharpness.  This 
is  quite  sufficient  to  prove  that  oilsands  differ  considerably  in 
the  matter  of  the  shape  and  contour  of  grains,  but  in  this 
particular  case  the  writer  is  unable  to  agree  with  either  Mr.  A. 
Beeby  Thompson  or  Professor  Clifford  Kichardson  on  the 
evidence  which  they  have  brought  forward  and  figured.  The 
Baku  sands  are  neither  so  well-rounded  nor  so  evenly  graded 
as  typical  wind-blown  sands;  it  is  more  probable  that  any 
special  degree  of  smoothing  or  rounding  which  these  grains 
exhibit  is  due  to  the  attrition  they  must  necessarily  have 
experienced  in  the  well  before  being  brought  to  the  surface, 
In  a  flowing  oil  well,  where  sand  is  brought  up  with  the  oil, 
there  must  be  a  great  churning  up  of  the  siliceous  material, 


52  OIL-FINDING 

quite  sufficient  to  add  a  polish  to  the  grains.  Mr.  Thompson's 
further  suggestion  that  the  sharpness  of  these  grains  may  have 
caused  an  epidemic  and  rapidly  fatal  disease  among  shoals  of 
supposititious  fish,  the  carcases  of  which  provided  the  raw 
material  for  the  formation  of  petroleum,  hardly  bears  out  his 
contention  as  to  the  sands  being  wind-blown. 

In  the  case  of  the  La  Brea  oilsand,  the  grains  are  certainly 
neither  so  sharp  nor  so  distinctly  broken  fragments  of  crystals 
as  to  suggest  deposition  from  solution.  It  is  a  very  ordinary 
water-borne  sand. 

There  is,  however,  no  reason  why  wind-blown  sands  or  any 
other  kind  of  sand  should  not  become  impregnated  with  oil: 
any  porous  rock  will  serve. 

Porosity. — Porosities  naturally  vary  very  greatly  in  oil- 
bearing  strata,  and  as  it  is  on  the  thickness  of  an  oilrock  and 
its  porosity  that  production  ultimately  depends,  the  subject  is 
worthy  of  careful  study. 

The  voids  in  a  rock  may  be  as  much  as  40  per  cent,  by 
volume,  but  that  is  exceptional  and  unlikely  to  be  met  with. 
Percentages  of  20  and  25  of  voids,  however,  are  not  by  any 
means  rare  occurrences  in  sands,  and  given  that  the  voids 
are  completely  filled  with  oil,  a  prolific  production  may  be 
expected.  Too  porous  an  oilsand  has  its  disadvantages,  since 
on  being  struck  in  a  well  the  cohesion  of  the  stratum  is  liable 
to  be  completely  broken  down,  and  quantities  of  sand  brought 
up  with  the  oil.  This  may  cause  choking  of  the  casing,  the 
wearing  away  of  flow-heads  and  caps  put  on  to  check  or 
control  the  flow,  and  in  extreme  cases  the  derrick  may  even  be 
half  buried  in  sand. 

In  Baku  the  quantity  of  sand  brought  up  by  flowing  wells 
has  been  a  cause  of  great  trouble  and  expense,  and  in  California 
and  Trinidad  similar  difficulties  have  been  encountered.  The 
oilsands  of  the  latter  colony  have  been  analysed  by  Professor 
Carmody,  Government  Analyst  and  Director  of  the  Agricultural 
Department,  samples  being  taken  from  outcrop  for  the  purpose. 
Percentages  by  weight  of  from  15  to  18  of  inspissated  petroleum 
have  been  recorded  from  outcrops  of  the  Eio  Blanco  oilsand. 
By  volume  this  would  mean  nearly  three  times  as  much,  so  it 
may  readily  be  understood  that  in  some  cases  the  surface  of  the 
outcrop  actually  shows  signs  of  flow.  Strata  in  this  case  cannot 
be  broken  by  a  hammer,  being  too  soft,  but  small  fragments 


SUBTERRANEAN    STORAGE  53 

can  be  twisted  off  in  the  fingers  and  rolled  into  pellets.  Where 
the  oil  is  not  inspissated  the  percentage  both  by  volume  and 
weight  will  not  be  so  high,  but  it  is  evident  that  strata  so  rich 
in  oil  will  break  down  easily  when  struck  in  a  well.  The 
experience  of  those  companies  who  have  drilled  into  the  Rio 
Blanco  oilsands  is  that  sand  is  always  tending  to  fill  up  part 
of  the  casing,  and  the  wells  must  be  constantly  cleaned  if  their 
production  is  to  be  maintained. 

The  La  Brea  oilsands,  the  youngest  oil-bearing  rocks  of 
Trinidad,  also  contain  a  very  large  percentage  of  petroleum, 
and  it  does  not  require  the  experience  of  drilling  near  the  Pitch 
Lake  to  prove  that  the  cohesion  of  the  rock  is  very  easily 
broken  down ;  the  Lake  itself  is  sufficient  evidence.  The 
winning  of  oil  from  wells  drilled  to  this  oil-bearing  group  will 
always  be  subject  to  great  difficulties  on  account  of  clogging 
of  the  casing  by  heavy  oil  and  sand,  and  the  wells  will  require 
constant  cleaning  out. 

The  most  satisfactory  oilsands  are  those  which  are  sufficiently 
compact  to  maintain  their  cohesion  even  when  the  well  is 
flowing.  The  greatest  productions  are  not  obtained  from  such 
sands,  but  wells  drilled  into  them  have  a  longer  life  and  are 
worked  much  more  economically. 

Paraffin  oils,  perhaps  because  they  are  as  a  rule  of  lighter 
gravity  than  asphaltic  oils,  seem  to  disengage  themselves  more 
easily  from  the  sands,  and  do  not,  as  a  rule,  carry  so  much 
sand  with  them.  This,  however,  may  be  partly  due  to  incipient 
paraffin ation,  the  deposit  of  paraffin  scale  in  the  sand  helping 
to  maintain  the  cohesion  of  the  rock.  The  sudden  relief  of 
pressure  and  consequent  lowering  of  temperature  when  a  prolific 
well  is  brought  in  in  a  paraffin-base  oilsand  must  cause  solid 
paraffin  to  be  deposited.  If  a  well  in  such  circumstances  be 
not  carefully  looked  after,  its  life  maybe  shortened  considerably 
by  the  sand  near  the  bottom  of  the  bore-hole  becoming  com- 
pletely clogged  with  solid  paraffin. 

The  great  advantage  that  limestone  has  over  sandstones  as 
an  oil  reservoir  is  that  it  does  not  break  down  and  choke  the 
bore-hole,  and  another  advantage  of  almost  equal  importance  is 
that  it  is  possible  to  torpedo  a  limestone  well,  the  production  of 
which  has  fallen  off  badly,  by  exploding  a  charge  of  nitro- 
glycerine at  the  bottom  of  the  bore.  This  usually  results  in 
giving  the  well  a  new  lease  of  life.  It -is  seldom  of  any 


54  OIL-FINDING 

use  to  torpedo  a  well  in  sandstone,  even  if  the  rock  be  fairly 
hard. 

Oil  occurs  not  infrequently  in  shales  and  clays  where  they 
have  some  degree  of  porosity,  but  the  yield  of  wells  drilled 
into  such  strata  is  always  small  and  the  petroleum  accumulates 
very  slowly.  The  oil  is  naturally  well-filtered,  and  light  in 
such  conditions,  but  production  is  seldom  sufficient  to  ensure 
a  commercial  success.  In  Java  wells  have  been  drilled  into  oil- 
bearing  shales,  yielding  an  excellent  oil,  but  not  in  sufficient 
quantity. 

There  is  some  evidence  suggesting  that  an  oil  may,  under 
certain  conditions,  prepare  its  own  reservoir  by  the  removal  in 
solution  of  cementing  material  in  a  rock.  This  probably  applies 
only  in  the  case  of  calcareous  cement,  and  may  take  place  only 
within  the  zone  of  weathering,  but  as  that  zone  may  extend 
downwards  for  some  hundreds  of  feet  the  results  might  be 
important. 

On  the  southern  coast  of  Trinidad  there  are  many  sections 
where  the  cliffs  have  been  cut  back  by  marine  denudation, 
leaving  a  very  gentle  slope  of  clays  usually  much  land-slipped 
— a  plan,  in  fact,  of  former  landslips.  The  strata  dip  steeply, 
and  contain  numerous  thin  beds  of  calcareous  sandstone  which 
stand  out  in  lines  above  the  clay  surface,  but  are  often  dis- 
continuous. 

These  sections,  though  washed  twice  a  day  by  the  tide,  reek 
with  the  odour  of  petroleum,  and  a  close  examination  shows 
that  similar  small  reefs  of  brown  oilsand  are  contained  in  the 
clay.  These  oilsands  are  seldom  more  than  a  foot  or  two  thick, 
and  they  resemble  the  calcareous  sandstone  reefs  in  every  way, 
except  that  they  contain  little  or  no  lime,  and  are  very  much 
softer  and  consequently  less  prominent.  They  are  quite  full 
of  petroleum  which  exudes  steadily  and  slowly,  forming  films 
upon  the  pools  of  water  left  by  the  receding  tide.  The  oil  is 
light,  and  is  accompanied  by  very  little  gas. 

Washed  about  on  the  shore,  and  sometimes  embedded  in 
the  clays  near  the  small  reefs  of  sandstone,  are  large  botryoidal 
masses  of  calcium  carbonate.  These  masses  are  dark  in  colour 
owing  to  the  inclusion  of  a  proportion  of  clay,  but  the  calcite 
is  well  crystallized,  the  crystals  radiating  from  the  centre  of 
each  rounded  mass.  These  botryoidal  masses  are  quite  different 
from  the  ordinary  fine-grained  calcareous  concretions  of  the 


SUBTERRANEAN    STORAGE  55 

clay.  They  occur  in  many  parts  of  the  island,  but  always  near 
the  outcrops  of  oil-bearing  strata.  Unfortunately,  they  are 
generally  found  loose,  washed  out  of  the  clay. 

In  one  locality  near  Galfa  Point  in  the  Cedros  district,  a 
bed  of  sandstone  some  six  feet  thick  is  exposed  among  clays 
on  the  foreshore.  Part  of  it  is  hard  and  calcareous,  and  part 
comparatively  soft,  brown  in  colour,  and  highly  petroliferous. 
In  the  calcareous  portion  petroleum  is  only  seen  along  joints 
and  bedding  planes.  The  calcareous  cement  does  not  occur 
like  a  concretionary  mass,  but  is  quite  irregular  in  outline  and 
appears  as  if  it  had  been  attacked  and  eaten  into  by  the  petro- 
liferous portion  of  the  rock.  Botryoidal  masses  of  calcite  are 
present  close  at  hand,  washed  out  of  the  clay  series. 

The  suggestion  is  made  that  these  botryoidal  masses  repre- 
sent calcite  that  has  been  dissolved  out  of  the  sandstone  and 
has  crystallized  out  in  the  softer  clay,  thus  leaving  room  for 
the  oil  to  impregnate  the  sandstone  beds.  In  the  zone  of 
weathering,  carbon  dioxide  and  water  might  be  present  in 
sufficient  quantity  to  attack  a  calcareous  cement,  but  the  action 
must  have  taken  place  beneath  the  surface  to  allow  the  dis- 
solved calcite  to  concentrate  under  concretionary  action  and 
crystallize  out. 

What  part  the  petroleum  and  its  accompanying  gases  can 
have  taken  in  such  an  action  it  is  difficult  to  determine ;  with 
the  help  of  water  they  may  have  supplied  the  corrosive  solution. 
The  point  to  be  noted  is  that  these  phenomena  have  only  been 
observed  where  oil-bearing  strata  are  present.  Further  study 
of  such  evidence  may  throw  light  upon  the  movements  and 
storage  of  oil,  and  especially  upon  the  effect  of  oil  and  water 
in  combination  upon  limestones,  and  may  help  to  explain  the 
selection  of  beds  to  form  oil  reservoirs,  even  when  they  are 
surrounded  with  almost  impervious  strata. 

There  are  many  minor  points  with  regard  to  the  under- 
ground storage  of  petroleum  which  might  be  cited,  but  all 
depend  upon  the  principles  already  laid  down,  the  selection  of 
the  most  porous  or  potentially  most  porous  stratum  available. 
The  migration  through  practically  impervious  beds  must  be 
very  gradual,  but,  given  sufficient  pressure,  it  is  sure,  though 
it  is  probably  only  the  lighter  constituents  of  the  mixed  hydro- 
carbons that  are  able  to  migrate  for  any  considerable  distance. 


CHAPTER  IV 
LATERAL   VARIATION 

HAVING  now  considered  most  of  the  more  important  theoretical 
questions  concerned  with  the  formation,  migration  and  storage 
of  petroleum,  let  us  turn  to  the  more  practical  matters  of  how 
oilfields  are  to  be  found,  and  how  we  can  make  as  sure  of  them 
as  possible.  In  the  next  five  chapters  facts  as  discovered  and 
studied  in  the  field  will  be  considered,  and  theory  as  far  as 
possible  eschewed,  while  methods  of  approaching  the  various 
problems  which  have  been  found  of  value  by  the  writer  will  be 
discussed. 

The  geologist  whose  task  it  is  to  prospect  a  new  country,  or 
a  new  area  in  a  well-known  country,  for  petroleum,  will  do  well 
to  prepare  himself  by  the  collection  of  as  many  previously 
known  facts  as  he  can  find  bearing  upon  the  particular  area, 
and  by  the  deliberate  abstention  from  reading  any  opinions, 
generalizations,  or  theoretical  matter  that  have  been  published^ 
about  it.  By  this  the  intention  is  not  to  cast  aspersions  upon 
any  work  done  previously  by  explorers,  geological  surveyors,  or 
travellers  with  a  taste  for  science,  but  simply  to  enable  the 
"  field-student "  to  start  work  with  a  perfectly  open  mind.  The 
line  between  opinion  and  fact  must  be  drawn  rigidly.  There  are 
very  few  countries  nowadays  which  are  not,  at  least,  partially 
known  geologically,  and  geological  surveys,  even  if  only  of  a 
pioneer  type,  have  done  much  excellent  and  sometimes  even 
detailed  work  in  many  parts  of  the  world ;  but  the  generaliza- 
tions into  which  the  pioneer  geologist  is  inevitably  tempted  are 
dangerous  things,  and  lest  they  should  impress,  oppress,  or 
antagonize  his  mind,  the  field-student  will  do  well  to  know 
nothing  about  them.  Ready-made  generalizations  fit  the  facts 
no  better  than  ready-made  coats  fit  the  body ;  they  are  the  bane 
of  original  work,  and  unless  the  observer  can  improve  upon 

56 


LATERAL  VARIATION  57 

what  has  been  done  before,  and  can  see  a  little  deeper  into  the 
geological  puzzles  that  await  him  than  has  been  done  by 
previous  workers,  he  is  unworthy  of  his  task. 

To  get  at  recorded  facts,  however,  without  absorbing  opinions 
is  a  matter  of  difficulty,  but  for  this  reason  the  writer  would 
emphasize  all  the  more  the  necessity  of  an  open  mind.  After 
field-work  has  been  done,  new  facts  collected  and  correlated, 
new  areas  mapped,  then  comes  the  time  for  reading,  for  testing 
theories  and  opinions  in  the  light  of  new  discoveries,  and  one's 
own  theories  in  the  light  of  how  such  or  similar  facts  appeared 
to  trend  in  the  minds  of  others. 

Let  a  small  scale  geographical  map  of  the  country  be 
procured  (if  there  be  such  a  thing  as  a  geological  map  it  will 
also  be  necessary),  and  let  the  prospector  sit  down  before  them 
and  study  them,  noting  roughly  on  each  such  essential  facts  as 
the  ascertained  or  reported  occurrences  of  surface  indications  of 
oil.  If  only  an  unknown  or  unprospected  district  of  a  country 
is  to  be  examined,  a  map  of  the  said  district  will  not  suffice  ; 
a  map  of  the  whole  country,  perhaps  with  portions  of  neigh- 
bouring countries,  is  essential.  The  "  field-student,"  as  the 
writer  prefers  to  call  one  who  reads  the  rock  rather  than  the 
printed  page,  who  travels  through  countries  rather  than 
reference  libraries,  is  now  in  search  of  a  few  general  ideas. 
What  if  they  prove  wrong  ?  It  is  no  matter ;  they  will  be 
^> tested  in  the  field. 

The  orientations  and  extents  of  the  principal  mountain 
granges,  the  courses  of  the  main  rivers,  and  the  character  and 
configuration  of  the  coast-line,  if  any,  are  naturally  the  first 
points  to  be  noted.  The  first  of  these  will  probably  give  a  very 
clear  indication  of  the  directions  of  the  principal  earth-move- 
ments to  which  the  area  has  been  subjected,  or  at  least  will 
show  that  one  of  two  directions  at  180  degrees  is  the  main 
direction  of  the  principal  or  latest  movement. 

The  courses  of  the  rivers  can  as  a  rule  be  divided  into 
"  consequent "  and  "  subsequent "  portions,  and  will  thus  in 
connection  with  the  mountain  chains  afford  considerable  assist- 
ance in  determining  roughly  the  main  strike-lines  of  the 
country. 

A  study  of  the  coast-line  should  indicate  what  parts  are 
rocky  and  what  parts  flat  and  low-lying,  and  the  presence  of 
any  delta  of  considerable  size  will  be  detected  at  once. 


58  OIL-FINDING 

If  the  oilfields  to  be  searched  for  or  examined  are  in 
Tertiary  strata,  the  methods  of  arriving  at  general  ideas  are 
simple,  as  it  is  only  the  latest  earth-movements  that  have  to  be 
considered.  If  series  older  than  the  Tertiaries  are  to  be 
examined  the  enquiry  becomes  more  complicated,  and  ifc  may 
not  be  possible  to  arrive  at  any  general  ideas  of  importance  by 
a  preliminary  study  of  the  map,  unless  some  geological  data 
are  available.  Most  of  the  world's  great  oilfields,  however,  are 
in  Tertiary  strata,  and  of  oilfields  yet  to  be  discovered  in  such 
countries  as  Galicia,  Roumania,  Eussia,  Egypt,  Turkey,  Persia, 
Baluchistan,  India,  China,  Venezuela,  Columbia,  Brazil, 
Argentina,  and  Mexico,  we  may  safely  assume  that  very  few, 
if  any,  will  be  in  rocks  older  than  the  Cretaceous  formation ;  so 
for  the  present  let  us  consider  that  a  Tertiary  Series  is  to  be 
prospected. 

Some  general  ideas  as  to  the  probable  main  structural  lines 
of  the  country  having  been  obtained  from  the  map,  and  the 
approximate  positions  of  known  and  reported  indications  of 
oil  noted  on  it,  the  prospector  must  ask  himself  why  the  oil 
is  found  in  such  localities  and  how  it  got  there.  These  queries 
may  take  long  to  answer,  or  to  obtain  any  light  upon;  when 
they  have  been  answered,  the  prospector  will  be  in  a  position 
to  determine  where  else  petroleum  is  likely  to  be  found.  The 
reason  for  considering  such  queries  and  attempting  to  find 
answers  to  them  is  that  the  general  question  of  the  occurrence 
of  petroleum  should  not  be  lost  sight  of  when  practical  field 
work  is  begun.  To  search  for  favourable  structures  in  areas 
which  are  apparently  outside  the  belt  of  country  in  which  it 
is  possible  to  find  oil  in  paying  quantity  is  not  only  a  waste 
of  time  from  the  practical  point  of  view,  but,  if  experimental 
wells  be  drilled  as  the  result  of  the  prospecting  work,  other 
instances  will  be  added  to  the  long  list  of  failures  which  have 
made  the  general  public  look  upon  oilfield  work  as  on  much 
the  same  level,  in  regard  to  risk,  as  gold  mining. 

The  prospector  is  now  ready  to  familiarize  himself  with  the 
lithological  characters  of  the  rock  with  which  he  has  to  deal. 
For  this  purpose  several  lengthy  traverses  across  the  main 
strike-lines  of  the  country  are  necessary,  and  also,  if  possible, 
one  or  more  roughly  along  the  strike.  The  object  is  not  only 
to  study  the  series  as  a  whole,  but  to  determine,  if  possible 
the  direction  or  directions  of  lateral  variation. 


LATERAL  VARIATION  59 

This  is  primarily  a  more  important  matter  than  the  study 
of  structure,  and  accordingly  it  is  considered  first.  In  the 
author's  experience  are  only  too  many  instances  of  the  follow- 
ing of  structure  in  oil  development  work,  while  the  lateral 
variation  in  the  strata  was  neglected  or  lost  sight  of. 

In  many  cases  the  working  out  of  the  directions  of  variation 
in  the  field  maybe  a  laborious  task,  necessitating  the  determina- 
tion of  the  stratigraphical  relations  of  different  groups,  but  in 
some  cases  a  clue  may  be  furnished  at  once  from  the  preliminary 
study  of  the  map.  There  may  be  a  great  river  in  the  country 
with  a  well-marked  delta,  and  the  evidence  may  point  to  this 
river  having  been  represented  in  Tertiary  times,  while  the 
ascertained  main  directions  of  earth-movement,  or  earth-waves, 
may  indicate  in  what  direction,  laterally  or  otherwise,  the 
course  of  the  river  has  probably  been  changed  between  Tertiary 
times  and  the  present  day.  In  Persia,  Burma,  and  Baluchistan, 
this  method  of  approaching  the  subject  proved  of  great  value. 

Deltaic  Conditions. — If  deltaic  or  estuarine  conditions  on  a 
large  scale  can  be  proved  to  have  occurred  during  the  Tertiary 
Series  in  question,  rapid  and  remarkable  variations  both  along 
and  across  the  main  strike-lines  are  almost  certain  to  be 
revealed.  The  field-student  must  look  for  constantly  alter- 
nating types  of  deposit,  e.g.  shales  or  clays  alternating  with 
sands.  Beds  of  undoubtedly  marine  origin,  fine  clays,  marls 
and  true  limestones,  must  be  differentiated  from  literal  or 
deltaic  deposits.  In  every  case  when  examining  a  bed  the 
geologist  must  consider  under  what  conditions  it  has  been 
formed.  The  only  satisfactory  method  of  arriving  at  a  con- 
clusion on  such  a  point  is  to  consider  under  what  conditions 
he  has  seen  similar  beds  being  formed  at  the  present  day,  and 
failing  such  direct  evidence  from  his  own  experience,  he  must 
consider  under  what  possible  conditions  could  such  a  bed  be 
formed.  In  such  an  enquiry  there  is  no  piece  of  evidence  that 
is  too  insignificant  to  note  down.  It  may  be  that  long  afterwards 
much  importance  is  found  to  attach  to  items  of  information 
jotted  down  in  note  books,  or  better  still  on  the  field  maps, 
items  which  at  the  time  seemed  to  be  entirely  insignificant 
details.  The  presence  of  gypsum  or  selenite  in  the  clays,  of 
glauconite  in  the  sands  or  argillaceous  sands,  and  of  remains 
of  terrestrial  vegetation  in  any  bed,  must  always  be  noted. 
These  all  point  to  estuarine  or  deltaic  conditions. 


60  OIL-FINDING 

In  a  general  way  the  main  directions  of  lateral  variation 
may  be  indicated  from  the  very  start  by  the  records  of  former 
observers,  e.g.  the  presence  of  thick  clays  or  limestones  in  one 
district,  and  of  coals  or  lignites  in  the  same  series  in  another, 
at  once  suggests  that  some  variation  may  be  expected  in  such 
and  such  a  direction,  even  though  the  horizons  of  the  particular 
deposits  have  not  been  ascertained. 

In  the  deltas  of  great  rivers,  channels  are  continually 
changing  their  courses,  so  that  sand-bearing  currents  trespass 
upon  mud-flats,  and  the  coarser  and  more  arenaceous  detritus 
thus  alternates  with  the  finer  and  more  argillaceous.  Sand- 
bars are  continually  being  formed  between  sea  and  delta, 
cutting  off  lagoons  or  salt  swamps.  These  sand-bars  also  are 
subject  to  sudden  modifications  through  the  action  of  tides  and 
currents :  they  may  be  extended  and  increased,  pushed  forward 
or  thrown  back,  cut  off  to  form  shoals  or  completely  swept 
away.  And  with  them  the  fate  of  the  lagoons  which  they 
protect  is  inseparably  bound  up.  They  may  be  filled  up  with 
detritus  to  form  solid  ground,  or  may  pass  through  a  stage  of 
mangrove  swamp  to  become  a  forest-lagoon,  a  forest  growing  at 
or  even  under  sea-level,  where  terrestrial  vegetation  flourishes, 
dies  and  accumulates  in  masses  which,  under  favourable  con- 
ditions, will  in  time  be  represented  by  coal  or  lignite  seams  or 
petroleum.  Any  slight  set-back  in  deposition,  any  temporary 
gain  of  subsidence  against  sedimentation,  and  the  lagoon  will 
be  invaded  by  the  sea,  the  vegetation  killed,  though  perhaps  not 
washed  away,  and  marine  sediments  may  be  deposited  above 
the  remains  of  forest  or  swamp  growth. 

Speaking  generally,  however,  a  delta  is  always  advancing 
in  one  direction,  in  spite  of  the  many  deflections  of  the  main 
river-channels.  A  delta  in  fact  means  a  victory  of  sedimenta- 
tion over  subsidence,  and  in  any  area  where  deltaic  conditions 
can  be  proved  to  have  existed  for  a  long  period,  littoral  sediment 
will  be  found  to  have  advanced  over  more  purely  marine  or 
pelagic  deposits.  Set-backs  no  doubt  frequently  occur,  owing 
to  periods  of  more  rapid  subsidence,  but  a  delta  stands  for 
continuous  deposition,  and  till  checked  by  a  movement  of 
upheaval  which  is  sufficient  to  enable  the  river  to  denude  its 
own  deposits,  or  by  the  encounter  with  powerful  ocean  currents, 
it  must  continue  to  advance. 

In   such   circumstances   it  is   obvious   that   rapid   lateral 


LATERAL   VARIATION  61 

variation  must  occur  somewhere  at  every  horizon.  In  some 
cases  the  variation  is  very  remarkable.  The  Tertiary  Series  in 
Trinidad,  formed  as  it  is  largely  of  fluviatile,  deltaic,  and 
estuarine  deposits  at  the  mouth  of  a  great  river,  which  is  now 
represented  by  the  Orinoco,  affords  some  very  striking  instances. 
The  island  is  situated  on  the  margin  of  a  continent  with  the 
deep  Atlantic  basin  not  far  to  the  eastward,  and  the  strata 
in  the  Tertiary  Series  represent  a  continual  struggle  between 
pelagic  and  deltaic  strata,  with  the  latter  gradually  becoming 
predominant,  and  variation  on  the  same  horizon  is  remarkably 
well  shown.  Thus  near  the  Cunapo  lignite  field  it  is  possible 
to  pass  on  the  same  line  of  strike  from  a  lignite  seam  through 
conglomerates  and  sands  representing  a  littoral  deposit  into 
muds,  fine  clays,  and  finally  a  marine  limestone  within  a 
distance  of  three  or  four  miles.  The  lateral  variation  in  that 
island  is  very  complicated,  and  has  not  been  fully  worked  out 
on  all  horizons  as  yet,  but  it  seems  to  have  been  lost  sight  of 
by  many  of  the  energetic  oil-prospectors  who  have  visited  the 
Colony. 

In  examining  a  deltaic  formation,  then,  variation  in 
almost  every  direction  may  be  observed  locally,  but  the 
algebraic  sum  of  all  variations,  supposing  it  were  possible  to 
measure  these  effects,  would  point  to  some  general  direction. 

The  concrete  bits  of  evidence  to  be  looked  for  are  the 
splitting  up  and  thinning  out  of  sandstone  beds,  the  decrease 
in  coarseness  of  arenaceous  sediment,  the  passage  of  sandstones 
into  thin  calcareous  sandstones  among  argillaceous  rocks  or 
finely  laminated  alternations  of  sand  and  clay,  the  oncoming  of 
finely  laminated  clays  without  gypsum,  and  the  directions  in 
which  they  thicken.  Similarly  the  development  and  thickening 
of  beds  of  calcareous  marl,  whether  foraminiferal  or  not,  and 
the  first  signs  of  true  limestone  bands  must  be  noted.  A  shell- 
bank,  formed  of  a  mass  of  broken  shells  on  a  shore  line,  must 
not  be  considered  as  a  limestone,  even  though  it  may  be  com- 
posed almost  entirely  of  carbonate  of  lime  ;  it  has  been  formed 
in  the  same  manner  as  a  littoral  sand.  Again,  the  thinning 
and  splitting  up  of  lignite  seams  among  banks  of  sand  and 
conglomerate,  which  were  the  bars  between  sea  and  lagoon, 
the  passage  of  such  seams  into  carbonaceous  sands  or  clays, 
and  the  passage  of  shales  into  underclays  and  leaf  beds,  are 
of  great  importance.  All  these  phenomena,  if  observed  carefully, 


62  OIL-FINDING 

will  give  definite  information  as  to  which  side  the  land  lay  and 
which  side  was  open  sea. 

All  evidence  of  shallow  water  conditions  or  sub-aerial 
conditions  such  as  false-bedding,  ripple  mark,  sun-cracks,  rain- 
pittings  on  fine  sands  and  clays,  and  in  some  countries  deposits 
of  lateritic  type,  which  were  weathered  and  oxidized  at  the 
time  of  formation,  and  represent  what  were  at  one  time  land- 
surfaces,  are  of  value. 

The  directions  of  currents  can  frequently  be  made  out 
from  the  arrangement  of  the  longer  axes  of  the  pebbles  in  a 
conglomerate,  and  especially  in  clay-gall  beds,  and  what  have 
been  called  "  clay  conglomerates,"  which  consist  of  pebbles  of 
more  or  less  soft  argillaceous  beds  in  a  sandy  matrix.  This 
type  of  deposit  is  caused  by  a  sand-bearing  current  impinging 
upon  a  partially  consolidated  clay  or  mud  deposit  and  breaking 
up  the  bed,  rolling  the  fragments  into  pebbles,  and  often 
bending  the  pebbles  so  formed.  They  pass,  by  the  gradual 
decrease  in  the  size  of  the  argillaceous  fragments,  into  sandy 
clays.  Beautiful  examples  of  this  type  of  deposit  can  be 
seen  on  the  western  coast  of  Trinidad,  and  may  be  photo- 
graphed in  cliff  sections,  where  the  actual  initial  bending 
up  and  breaking  of  the  argillaceous  bed  is  sometimes  observed, 
the  current  action  having  been  checked  exactly  at  this  stage. 
In  Burma  also,  and  in  Persia,  where  the  detrital  limestones 
have  thinned  out  and  become  muddy  and  sandy,  bands  formed 
in  this  manner  are  to  be  seen. 

Finally,  fossil  evidence  must  be  studied  in  connection  with 
these  variations,  but  the  fossils  must  not  be  taken  from  the 
mixed  faunas  formed  in  littoral  beds,  where  specimens  from 
littoral,  laminarian,  and  pelagic  zones  are  washed  about  on 
the  beach  together,  but  from  the  actual  deposits  in  which 
or  on  which  the  organisms  lived.  Oyster  beds,  for  instance, 
may  be  noted  as  important ;  foraminiferal  beds,  thick  clays 
containing  lamellibranchs  with  joined  and  closed  valves  and 
gasteropods  in  perfect  preservation,  and  assemblages  of  fresh 
and  brackish  water  forms  are  all  of  help  in  determining 
directions  of  variation. 

In  well-exposed  sections  on  river  banks,  sea  cliffs,  road 
or  railway  cuttings,  and,  if  the  ground  be  not  too  much 
obscured  by  vegetation,  in  any  hilly  ground,  it  is  possible 
to  study  all  these  phenomena  and  to  derive  from  each  some 


LATERAL  VARIATION  63 

link  in  the  chain  of  evidence  as  to  the  side  on  which  sea 
and  shore  respectively  lay,  while  the  deposits  were  being 
formed. 

When  this  study  is  extended  over  wide  areas  and  over 
many  horizons  in  a  thick  series,  the  course  of  a  delta  can  be 
made  out  with  considerable  accuracy  at  each  successive  epoch, 
and  it  can  be  shown  to  have  pushed  forward  its  littoral  sedi- 
ment, now  rapidly,  now  slowly,  with  recurring  intervals  of 
retreat,  and  with  perhaps  slightly  diverging  directions  at 
different  times,  but  over  all  with  a  steady  inevitable  advance 
over  the  more  characteristic  marine  sediments  with  which  it 
was  contemporaneous. 

The  Pegu  Series  of  Burma,  ranging  from  the  Eocene  far  up 
into  the  Miocene  according  to  our  subdivisions  of  Tertiary 
time,  furnishes  perhaps  the  most  conclusive  evidence  of  the 
advance  of  a  delta  that  has  been  worked  out  in  any  detail. 
All  the  phenomena  of  deposition  mentioned  above  can  be 
studied  in  this  series,  but  for  the  most  part  the  thinning  out 
and  splitting  up  of  sandbeds,  and  the  simultaneous  thickening 
of  clays,  are  sufficient  to  make  the  directions  of  variation 
quite  clear,  so  that  we  are  now  enabled  to  elucidate  the  history 
of  the  series  in  almost  all  parts,  and  to  give  an  idea  of  the 
conditions  under  which  each  particular  bed  was  formed.  The 
boring  journals  of  oil  wells  have  been  of  the  very  greatest 
assistance  in  establishing  the  history  of  the  Pegu  Series  point 
by  point. 

A  great  river  flowing  from  the  northward  entered  a  land- 
locked gulf,  and  hugging  the  western  shore,  gradually  filled 
it  up  by  its  advance.  Much  of  the  axis  of  the  Arakan  Yomas 
was  already  land  when  the  Pegu  Series  began  to  be  deposited, 
and  along  the  western  shore  thus  formed  great  littoral  sand- 
stones with  much  evidence  of  terrestrial  vegetation  were 
deposited.  On  its  eastern  side  the  deltaic  deposits  were  inter- 
calated with  truly  marine  beds.  The  advance  of  the  deltaic 
deposits  was  not  steady,  but  subject  to  many  checks  and 
retreats.  During  some  of  the  checks  vast  accumulations  of 
vegetable  matter  were  formed  in  the  swamps  and  lagoons  near 
the  river-mouth,  to  be  afterwards  buried  under  marine  deposits 
of  the  invading  sea  or  covered  by  coarser  estuarine  detritus 
as  the  delta  was  pushed  forward.  Thus  in  Lower  Burma  the 
oldest  strata  of  the  Pegu  Series  are  entirely  or  almost  entirely 


64  OIL-FINDING 

marine,  while  deltaic  and  even  terrestrial  conditions  existed 
simultaneously  in  parts  of  Upper  Burma.  Earth-movement 
was  in  evidence,  but  not  very  active. 

When  the  delta  passed  beyond  the  shelter  of  the  western 
coast  its  course  was  to  some  extent  deflected  by  ocean  currents, 
but  it  continued  to  advance  over  the  marine  sediment.  Now, 
after  many  changes  of  level,  and  the  deposit  of  an  overlying 
flu  via  tile  series  (which  is  usually  unconformable  to  the  Pegu 
Series,  but  also  has  a  marine  phase  that  cannot  but  be  conform- 
able somewhere  to  the  Pegu  Series),  the  Irrawaddy  has  fallen 
heir  to  the  former  great  river ;  and  though  it  has  a  somewhat 
different  course  the  general  direction  is  the  same,  and  we  can 
still  study  the  advance  of  a  delta  at  its  mouth. 

In  other  countries  the  advance  or  retreat  of  deltaic  fans  of 
detritus  may  also  be  proved,  but  few  cases  are  so  simple  as 
that  of  Burma.  In  Trinidad,  for  instance,  earth-movement  on 
a  fairly  large  scale  supervened  during  the  deposition  of  the 
Tertiary  Series  and  caused  certain  complications,  so  that  the 
upper  strata  lie  in  a  violent  unconformity  across  the  denuded 
strata  of  Middle  Tertiaries.  Yet  the  main  directions  of  lateral 
variation  can  be  proved  with  some  degree  of  accuracy.  During 
the  deposition  of  the  earliest  Tertiary  strata,  sedimentation 
was  advancing  from  the  south-east,  while  marine  conditions 
persisted  for  a  longer  period  in  the  south-west.  Towards 
the  middle  of  the  Tertiary  period  sediment  was  poured  in  from 
the  south  and  west,  and  the  arenaceous  detritus  is  intercalated 
with  and  passes  into  pelagic  strata  to  the  north  and  north-east. 
Then  followed  a  period  of  retreat  when  the  advancing  arms  of 
the  delta  barely  held  their  own,  and  fine  clays  and  forarniniferal 
marls  were  deposited  above  strata  of  deltaic  origin.  Finally, 
sedimentation  advanced  again,  and  arenaceous  strata  were 
deposited  by  many  currents  flowing  in  various  directions,  east 
and  west  and  north.  The  presence  of  islands  of  older  strata, 
and  the  inception  of  a  folding  movement  acting  in  a  northerly 
direction,  introduced  many  complications,  but  the  main  branches 
of  the  delta  can  be  followed  out,  and  seams  of  lignite  and  bands 
of  oil-bearing  rock  at  various  horizons  mark  approximately  the 
localities  where  accumulations  of  vegetable  matter  in  forest 
lagoons  or  swamps  were  formed. 

The  same  principles  may  be  applied  to  the  elucidation  of 
the  history  of  the  Tertiary  Series  in  many  other  countries,  but 


LATERAL  VARIATION  65 

it  would  weary  the  reader  to  enter  into  elaborate  details  of  the 
evidence  from  one  country  after  another  that  has  served  to 
confirm  the  theory  and  to  associate  oilfields  with  deltaic 
conditions. 

The  point  of  all  this  insistence  upon  the  importance  of 
studying  lateral  variation  and  determining  the  boundaries  of 
a  deltaic  formation  is  simply  that  the  probable  petroliferous 
belt  may  be  recognized.  If  oil  is  to  be  drilled  for,  it  is  as  well 
to  look  for  it  as  near  as  possible  to  areas  where  the  conditions 
for  its  formation  were  favourable.  On  the  one  hand  may  be 
littoral  and  terrestrial  beds  where  the  carbonaceous  phase  is 
in  evidence,  on  the  other  marine  beds  beyond  the  confines  of 
the  delta.  Somewhere  between  we  may  hope  for  an  area  where 
the  necessary  alternations  of  arenaceous  and  argillaceous 
sediment  are  present,  an  area  not  too  far  from  localities  where 
accumulations  of  vegetable  matter  have  probably  been  formed, 
buried,  and  sealed  up.  In  that  area  we  must  seek  for  favour- 
able structures  to  concentrate  and  retain  the  petroleum. 

Much  excellent  work  of  competent  geological  surveyors, 
much  arduous  toil  in  opening  up  new  districts  and  transporting 
plant  to  them,  much  fruitless  expenditure  of  money  and  time 
could  have  been  saved,  had  the  facts  concerning  lateral  varia- 
tion been  carefully  studied  and  mastered. 

It  is,  of  course,  a  commonplace  of  geology  that  lateral 
variation  occurs  in  rocks  of  all  ages,  but  unfortunately  in 
Britain,  the  birthplace  of  stratigraphy,  the  variations  in  most 
formations  and  series,  with  a  few  notable  exceptions,  are  not 
very  great,  and  much  correlation  of  strata,  it  is  to  be  feared, 
is  still  attempted  chiefly  on  lithological  grounds.  Where  we 
have  evidence  of  continuous  deposition  on  a  large  scale,  and 
deltaic  and  estuarine  conditions,  as  we  have  in  the  Carboniferous 
Formation,  it  has  taken  decades  of  field-work  and  controversy, 
and  volumes  of  scientific  papers  written  and  discussed  before 
it  has  been  possible  to  arrive  at  a  conclusive  general  idea  of 
the  conditions  and  variations.  Even  at  the  present  day  we 
find  a  classification  based  upon  subdivisions  established  in 
some  districts  in  England,  including  the  well-known  Millstone 
Grit,  forced  upon  Scotland  where  such  a  classification  is  neither 
natural  nor  of  practical  benefit;  and  still  " pakeontological 
breaks"  are  inserted  in  a  continuous  series  in  order  that  a 
universal  general  arrangement  may  be  adhered  to.  Similarly 

F 


66  OIL-FINDING 

we  have  seen  our  local  subdivisions  such  as  Eocene,  Oligocene, 
Miocene,  and  Pliocene  forced  upon  other  countries  where  their 
only  significance  is  chronological,  and  where  no  natural  group- 
ings can  be  made  to  coincide  with  them.  In  our  turn  we 
have  adopted  Continental  subdivisions,  e.g.  in  the  Cretaceous 
Formation,  which  are  at  the  least  very  doubtfully  applicable. 

But  if  lateral  variations  during  continuous  deposition  can 
be  proved  to  be  common  and  distinct  among  the  primary  and 
secondary  formations  in  the  Tertiaries  almost  all  over  the 
world,  they  become  even  more  frequent  and  impressive,  because 
it  is  those  Tertiary  strata  which  have  emerged  comparatively 
recently  from  beneath  sea-level  that  we  know;  the  great 
uniform  Tertiary  deposits,  which  perhaps  future  geologists  will 
examine,  are  still  beneath  the  waves.  That  is  to  say,  we  know 
only  the  margins  of  the  Tertiary  formations,  and  it  is  precisely 
along  land-margins  and  on  the  fringes  of  continental  areas 
that  lateral  variations  must  naturally  be  greatest. 

"We  must  take  variation,  then,  as  the  rule  and  not  the 
exception  when  studying  Tertiary  strata,  and  must  not  attempt 
the  correlation  of  distant  areas,  as  the  author  has  often  seen 
done,  by  similarity  of  lithological  characters  or  the  presence  of 
some  particular  mineral  or  minerals.  Such  evidence  only 
means  similarity  in  the  conditions  of  deposition,  a  similarity 
which  during  progressive  sedimentation  must  migrate  from  one 
area  to  another.  Thus  oil-forming  and  oil-bearing  conditions 
may  be  transferred  from  the  lower  beds  of  a  series  to  the  upper 
beds  as  we  proceed  from  one  province  to  another. 


CHAPTER  V 
GEOLOGICAL  STRUCTURE 

IT  will  be  noticed  that  what  is  usually  considered  the  most 
important  matter  in  oilfield  work,  the  study  of  geological 
structure,  is  given  a  secondary  place.  This  has  been  done,  not 
because  its  importance  is  not  fully  recognized,  but  because  the. 
study  of  lateral  variation  comes  first  naturally,  and  may  render 
unnecessary  a  great  deal  of  detailed  work  in  discovering  and 
mapping  favourable  structures.  The  field-student  has  now 
advanced  sufficiently  in  his  knowledge  of  the  country  which  he 
is  examining  to  be  able  to  predict  in  what  districts,  and 
possibly  also  at  what  general  geological  horizons,  conditions 
are  favourable  to  the  formation  of  petroleum.  His  next  task 
is  to  discover  in  the  indicated  districts  suitable  structures  to 
contain  and  preserve  the  petroleum  from  inspissation,  and  to 
ensure  sufficient  concentration  to  make  paying  productions 
probable. 

Earth-movements. — The  elucidation  of  geological  structure 
naturally  depends  on  a  study  of  the  earth-movements  that  have 
been  experienced.  Here,  again,  an  examination  of  the  general 
map  of  the  country  is  essential ;  it  is  advisable  to  get  a  broad 
view  of  such  evidence  of  folding,  faulting,  and  unconforma- 
bilities  as  has  been  obtained  before  details  of  structure  are 
attacked. 

In  the  preliminary  traverses  made  to  gain  an  insight  into 
the  lateral  variation,  the  geologist  has  doubtless  obtained  some 
evidence  of  folding  and  the  direction  of  folding  movements. 
In  many  cases  only  one  earth-movement  will  require  to  be 
considered;  in  others  two,  or  even  more,  of  different  ages, 
directions  and  degrees  of  severity  may  have  to  be  distinguished 
and  delineated.  Earth-movements  are  instances  of  relative 
movement,  but  as  a  rule  it  will  be  found  simpler  to  consider 
them  as  movements  in  a  definite  direction  towards  some  central 

67 


68  OIL-FINDING 

axis  of  folding,  the  force  being  applied  tangentially  to  the 
earth's  crust.  There  is  no  structure  produced  by  flexuring  or 
faulting  that  cannot  be  explained  by  the  application  of  this 
simple  principle. 

The  movement  to  be  considered,  then,  resolves  itself  into 
a  horizontal  push,  and  in  the  majority  of  cases  the  direction  is 
from  the  seaward  towards  the  mountain  ranges.  Again,  the 
oldest  strata  exposed  will  be  found  as  a  rule  in  the  heart  of  any 
axis  of  folding  that  may  be  present.  This  gives  another  method 
for  determining  on  prima  facie  evidence  the  direction  of  move- 
ment. 

Where  flexuring  attains  to  great  dimensions,  and  a  series 
of  well-marked  parallel  folds  is  produced,  the  steep  sides  of 
asymmetrical  folds  will  be  found  almost  invariably  on  the 
side  from  which  the  movement  took  place,  and  vertical  or  even 
inverted  limbs  of  flexures  are  not  uncommon  in  such  circum- 
stances, even  among  comparatively  young  Tertiary .  strata. 
This  is  all  in  accordance  with  the  development  of  a  geanticline, 
and  the  production  of  a  facher  or  fan  structure,  in  which  the 
axial  plane  of  the  central  fold  is  vertical  or  nearly  so,  and  the 
axial  planes  of  the  sharp  flanking  folds  dip  towards  the  central 
axis.  As  one. recedes  from  the  central  axis  of  folding,  the 
flexures  gradually  become  less  sharp  and  more  symmetrical, 
till  finally  they  may  be  represented  by  small  gentle  undulations 
or  elevations  which  affect  the  dip  of  the  strata  so  slightly  that 
the  pocket  clinometer  may  not  be  sufficiently  accurate  and 
sensitive  to  prove  a  general  dip  in  any  one  direction.  In  both 
Persia  and  Burma  there  are  excellent  examples  of  a  series  of 
flexures  rapidly  decreasing  in  sharpness  as  we  recede  from  the 
central  axis  of  folding. 

Thus  when  a  wide  area  is  to  be  examined  there  is  usually 
little  difficulty  in  determining  the  direction  of  movement,  and 
after  a  traverse  across  the  main  strike-lines  of  the  country  it 
should  be  possible  to  predict  where  gentle  folds  should  be  in 
evidence,  where  sharp  or  asymmetrical  flexures,  and  where 
inversions  of  the  steeper  limbs  of  flexures  may  be  expected. 

The  age  and  duration  of  the  earth-movements  is  another 
matter  of  great  importance.  In  some  cases  it  will  be  found  that 
movement  has  proceeded  fairly  steadily  during  the  deposition 
of  a  Tertiary  series,  causing  older  strata  to  be  elevated  on  the 
crests  of  flexures,  brought  into  the  reach  of  denuding  forces  and 


GEOLOGICAL  STRUCTURE  69 

actually  denuded,  while  continuous  deposition  was  proceeding 
in  the  synclines.  The  results  produced  are  violent  uncon- 
formities along  certain  lines,  with  the  unconformability  dying 
out  laterally,  while  the  older  strata  may  be  seen  in  localities 
sharply  folded  and  overlaid  conformably  by  successive  younger 
strata  in  which  the  dips  decrease  steadily  upwards,  the  upper- 
most beds  perhaps  being  practically  horizontal.  The  best 
instance  of  this  that  has  come  under  the  writer's  observation 
is  in  Persia  (cp.  Plate  VII),  where  the  movement  has  been  in 
operation  since  early  Miocene  times  and  is  probably  still 
continuing. 

In  other  cases  the  movement  may  have  been  long-continued, 
but  not  continuous,  so  that  at  several  distinct  epochs,  separated 
by  intervals  of  quiescence,  it  has  been  rapid.  The  results  will 
be  the  production  of  local  unconformabilities  at  different  stages, 
but  these  unconformabilities  may  die  out  laterally  within  a 
comparatively  short  distance,  and  must  not  be  treated  as  if 
they  were  universal.  Great  lateral  variations  in  the  strata 
will  be  caused  under  such  conditions.  Sind  and  Baluchistan 
afford  a  good  instance  of  this.  In  the  records  of  the  Geological 
Survey  of  India  dealing  with  this  province  it  will  be  seen  that 
a  great  number  of  types  of  sediment  are  represented,  and  very 
frequently  they  are  separated  by  unconformabilities.  The 
unconformabilities  in  this  case  are  almost  entirely  of  local 
importance  only ;  there  is  great  lateral  variation,  but  there  has 
been  little  denudation  of  previously  formed  beds  throughout 
the  series  from  early  Eocene  up  to  perhaps  middle  Miocene. 

In  other  cases  definite  periods  of  folding  movement  may  be 
made  out,  and  the  relative  ages  of  each  can  be  determined  by 
the  effects  upon  series  of  different  ages.  This  is  the  case  in 
Burma,  where  one  movement  has  been  detected  that  affects  the 
Pegu  Series  and  earlier  strata,  while  another  and  much  greater 
movement  in  a  different  direction  affects  not  only  the  Pegu 
Series  but  the  younger  Irrawaddy  Series  lying  unconformably 
upon  it. 

In  studying  earth-movement  it  must  be  remembered  that 
faults  are  part  of  the  movement  just  as  much  as  flexures.  A 
fault  is  merely  a  special  case  of  folding,  where  the  elasticity 
of  the  strata  or  the  amount  of  "load"  is  not  sufficient  to 
prevent  dislocation.  Theoretically  a  fault  may  always  be 
replaced  by  a  sharp  monoclinal  bend,  and  the  passage  from  one 


;0  OIL-FINDING 

into  the  other  may  often  be  seen.  It  has  too  often  been  the 
custom  to  think  of  a  fault  as  something  quite  apart,  and  to 
map  a  fault — to  use  a  metaphor  from  whist — as  one  would 
play  a  trump,  not  following  suit.  The  author  has  known 
faults  to  be  recklessly  strewn  about  a  geological  map  in  this 
manner,  when  the  presence  of  one  could  not  be  justified  without 
the  mapping  of  two  or  three  more  which  were  entirely  theo- 
retical, for  which  no  evidence  could  be  obtained,  and  of  which 
the  amounts  and  directions  of  throw  were  purely  conjectural. 
From  such  methods  it  soon  arrives  that  when  any  structure 
has  not  been  elucidated  properly,  the  remark  will  be  made 
"there  must  be  a  fault,"  and  the  puzzle  is  deemed  to  be 
explained.  A  fault  would  thus  become  a  sort  of  make-shiffc  to 
get  faulty  observers  out  of  trouble.  But  the  effect  is  just  the 
opposite ;  such  methods  soon  affect  their  own  cure  by  involving 
the  geological  surveyor  in  a  net-work  of  physical  impossibilities, 
from  which  the  only  escape  is  to  begin  the  mapping  all  over 
again. 

Faults  are  really  very  simple  matters,  and  they  obey 
physical  laws  just  as  folds  do.  They  must,  therefore,  only  be 
mapped  when  justified  on  physical  grounds,  and  no  fault  must 
be  recorded  of  which  at  least  the  direction  of  throw,  if  not 
some  estimate  of  the  actual  amount  of  throw,  can  be  given. 

Structures  Favourable  to  Concentration  of  Petroleum. — With 
these  preliminary  remarks  we  may  pass  to  a  consideration  of 
the  structures  most  favourable  to  the  underground  concentration 
of  petroleum.  It  is  usual  in  books  upon  the  subject  to  give  a 
list  of  the  various  structures  which  have  been  tested  and 
proved  productive,  and  a  great  number  of  different  classes  of 
structure  can  be  described.  But  any  one  can  be  assigned  to 
one  of  a  few  main  types.  Anticlinal  structure  and  petroleum 
are  associated  in  the  minds  of  all  who  have  studied  or  worked 
in  oilfields,  and  though  petroleum  can  be  proved  to  occur  in 
almost  every  known  structure,  in  the  vast  majority  of  cases 
some  form  of  anticline  is  present  in  a  successful  field. 

Dome  Structure. — It  will  be  admitted  generally  that  the 
most  favourable  structure  of  all  is  a  dome  or  quaquaversal, 
with  gentle  dips  near  the  summit  and  steeper  dips  upon  the 
flanks,  which  again  pass  gradually  and  steadily  into  a  position 
of  horizontality,  so  that  a  large  area  can  be  included  as  properly 


GEOLOGICAL  STRUCTURE  71 

belonging  to  the  dome.  This  is  merely  a  special  case  of 
anticlinal  structure,  an  anticline  with  pitches  of  the  axis  away 
from  a  central  point. 

A  broad  round  dome  is  very  rare  in  nature,  as  it  almost 
necessarily  requires  more  than  one  earth-movement  for  its 
formation.  Elongated  domes,  however,  are  fairly  common  and 
there  are  few  anticlines  that  have  not  in  one  or  more  localities 
some  trace  of  dome-structure.  Spindle-Top  is  probably  an 
isolated  dome  of  breadth  approximating  closely  to  its  length, 
though  the  evidence  does  not  seem  ever  to  have  been  recorded 
very  clearly.  The  famous  Yenangyoung  field  in  Burma  is 
an  elongated  dome  of  great  extent,  nearly  symmetrical,  and 
only  slightly  affected  by  purely  local  faults.  It  is  isolated 
from  any  other  flexure  by  miles  of  approximately  horizontal 
strata,  and  thus  drains  a  large  area,  while  steep  dips  occur 
on  either  flank,  ensuring  a  high  concentration  of  the  petroleum. 
About  two  square  miles  of  available  drilling  area  is  provided 
on  its  crest.  It  is  obvious  that  under  such  conditions  hydro- 
static pressure  of  water  in  the  strata  is  enabled  to  concentrate 
the  petroleum  from  all  sides  towards  the  summit.  In  such 
ideal  structures,  whatever  be  the  quality  of  the  oil,  and  however 
small  the  porosity  of  the  oil-bearing  strata,  such  a  concentration 
is  bound  to  take  place  and  large  productions  may  be  expected 
whenever  the  oilsands  attain  to  a  reasonable  thickness. 

Symmetrical  Anticlines. — Next  in  importance  comes  the 
simple  symmetrical  anticline  (cp.  Plate  VIII),  either  without 
pitches  of  the  axial  line,  or  with  pitches  too  low  to  have 
affected  the  structure  favourably  or  unfavourably.  Many  of 
the  Eastern  fields  in  the  United  States  have  structures  of  this 
nature,  the  anticlines,  though  extensive,  being  often  so  low 
and  flat  as  to  be  only  distinguishable  by  very  careful  levelling 
or  by  evidence  from  actual  bores.  It  is  obvious  that  the 
greater  the  extent  of  the  flexure,  the  greater  should  be  the 
concentration  of  oil  towards  the  crest,  given  sufficient  hydro- 
static pressure.  The  effects  of  pitches  and  gentle  dips  will 
depend  to  a  great  extent  on  the  nature  of  the  oil  and  the 
porosity  of  the  oil-bearing  strata ;  the  greater  the  specific  gravity 
of  the  oil  and  the  smaller  the  porosity,  the  slower  and  less 
complete  will  be  the  migratory  movement.  Thus  structures 
that  have  proved  remarkably  favourable  for  the  production 
of  a  light  oil  of  paraffin  base,  may  not  cause  any  great 


72  OIL-FINDING 

concentration  of  a  heavier  grade  of  petroleum.  In  the  United 
States  the  fields  of  New  York,  Ohio,  Pennsylvania  and  Virginia, 
where  as  a  rule  the  oil  is  light  and  mobile,  show  many  instances 
of  very  flat  structures,  anticlines  with  slopes  of  twelve  feet  in 
a  mile,  for  instance.  It  was  in  these  fields  that  drilling  for 
oil  was  first  attempted  and  learnt,  and  they  were  taken  as  the 
type  of  what  oilfields  should  be.  This  idea  still  survives  to 
some  extent  in  spite  of  the  discovery  of  so  many  great  fields 
with  totally  different  structures.  The  advantages  of  these 
eastern  fields  in  the  States  are  many ;  horizontal  or  low  dips 
make  the  easiest  drilling,  and  the  strata  being  palseozoic  are 
mostly  fairly  hard  and  not  very  liable  to  "  caving,"  so  that 
drillers  trained  in  these  fields  were  acquainted  with  few  of  the 
difficulties  attendant  on  drilling  in  soft  Tertiary  strata.  When 
oil  began  to  be  discovered  in  other  provinces  under  entirely 
different  conditions,  many  a  field  that  has  since  proved  very 
profitable  was  condemned  at  first  because  it  did  not  conform 
to  the  structural  peculiarities  deemed  essential  in  the  fields 
of  the  Eastern  States,  and  many  a  practical  operative,  with  only 
experience  of  the  Eastern  fields,  proved  a  failure  when  con- 
fronted with  the  task  of  drilling  through  soft  and  steeply 
dipping  Tertiary  strata. 

Asymmetrical  Anticlines. — Another  form  of  structure  that 
has  provided  many  excellent  fields  is  the  asymmetrical  anti- 
cline. This  is  an  anticline  with  one  flank  gently  and  the  other 
steeply  inclined ;  the  latter  in  some  cases  may  be  vertical  or 
even  inverted.  Such  flexures  usually  occur  nearer  to  the 
central  axis  of  folding  than  symmetrical  flexures.  The  "  terrace 
structure,"  well  known  and  much  sought  after  in  the  eastern 
fields  of  the  United  States,  may  be  regarded  as  a  special  case  of 
this  form  of  structure,  an  anticline  so  flat  and  gentle  that  the 
gently  dipping  flank  is  for  all  practical  purposes  horizontal. 
In  sharp  asymmetrical  anticlines  such  as  those  at  Kasr-i-Cherin 
in  Persia,  and  Yenankyat  and  Singu  in  Burma,  it  is  evident 
that  drilling  on  the  actual  line  of  crest  is  useless ;  the  drill 
soon  enters  steeply  dipping  beds  where  great  difficulties  may  be 
encountered  in  the  drilling,  while  there  may  be  no  possibility 
of  penetrating  to  a  sufficiently  low  horizon.  Mr.  G.  B.  Reynolds 
pointed  this  out  in  the  Persian  fields,  and  Mr.  Pascoe  in  the 
Eecords  of  the  Geological  Survey  of  India  has  since  explained 
the  effect  of  such  a  structure  in  the  case  of  the  Yenankyat  field 


GEOLOGICAL  STRUCTURE  73 

in  Burma.     This  point  will  be  referred  to  later  in  the  chapter 
on  "  Location  of  Wells." 

Compound  Anticlines, — Compound  anticlines  may  next  be 
considered.  These  are  only  observed  where  the  flexuring  move- 
ment has  been  severe,  and  has  produced  a  series  of  sharp  folds, 
possibly  with  very  steep  flanks.  The  most  striking  instance 
that  has  come  under  the  writer's  observation  is  at  Maidan-i- 
Naphtun,  in  Persia,  where  a  group  of  no  less  than  seven  sharp 
local  flexures  is  included  in  an  area  approximately  one  mile 
in  breadth.  Some  of  the  flexures  are  so  sharp  that  when  a  band 
of  hard  limestone  is  exposed  on  the  crest  it  is  possible  to  sit 
astride  on  the  anticline.  Highly  inclined  strata  are  the  rule 
throughout  this  field,  and  vertical  or  inverted  limbs  of  folds  are 
common.  These  seven  flexures  converge  towards  and  pass  into 
one  broader  fold,  which,  being  formed  of  strata  less  amenable  to 
distortion,  is  somewhat  gentler,  and  which  really  gives  the  key 
to  the  structure  of  the  neighbourhood.  The  whole  area  is 
broadly  anticlinal,  and  the  sharp  folds  are  merely  puckers  upon 
a  well-defined  flexure  on  a  larger  scale.  Every  well  drilled  in 
the  area,  whether  upon  one  of  the  minor  anticlines  or  in  one 
of  the  synclines,  has  struck  oil  in  paying  quantity,  but  the 
surface  indications  of  oil  are  nearly  all  upon  or  close  to  the 
crests  of  the  minor  flexures.  These  minor  folds,  though  very 
striking  and  impressive,  can  therefore  be  disregarded  and  the 
compound  anticline  considered  as  a  whole. 

Synclines. — Mr.  W.  T.  Griswold  has  pointed  out  that  under 
certain  conditions  synclines  may  be  exploited  for  oil  success- 
fully. The  necessary  conditions  are  that  the  strata  are  not 
waterlogged,  and  are  so  covered  or  sealed  that  water  cannot 
enter  the  oil-bearing  bands  at  outcrop.  In  such  circumstances 
any  petroleum  in  a  porous  rock  will  tend  to  collect  at  the 
bottom  of  the  syncline  under  the  force  of  gravity.  It  is  very 
doubtful  whether  many  such  cases  exist,  though,  in  rainless 
regions  or  where  the  bulk  of  the  strata  are  practically  im- 
pervious to  water,  such  conditions  are  possible. 

With  an  oil  of  approximately  the  same  specific  gravity  as 
water,  displacement  by  the  water  might  be  very  slow  and  never 
quite  complete :  so  a  certain  quantity  of  the  petroleum  might 
remain  in  synclines,  especially  if  deposits  of  asphalt  were 
formed  upon  the  outcrops  by  inspissation  of  the  exuding 


74  OIL-FINDING 

petroleum  and  the  downward  percolation  of  water  checked  if  not 
entirely  prevented.  Thus,  although  with  a  light  oil  it  may 
very  seldom  be  worth  while  to  drill  in  a  syncline,  with  heavy 
asphaltic  oils  and  in  regions  where  rainfall  is  very  small,  shallow 
synclines  might  be  worthy  of  being  tested,  and  might  prove 
highly  productive.  Even  in  Trinidad  where  the  rainfall  is  high 
there  is  some  evidence  in  favour  of  making  a  test  of  one  of  the 
shallow  synclines,  where  extensive  asphalt  deposits  cover  most 
of  the  outcrops  of  oil-bearing  sand.  Where  oils  are  very  heavy 
and  sluggish,  a  certain  proportion  of  water  in  the  oilrocks  is 
rather  a  benefit  than  otherwise,  assisting  the  flow  of  oil. 

Monoclines. — Finally,  oil  may  be  obtained  from  monoclines, 
often  in  great  quantity.  In  such  cases  the  more  gentle  the 
dip,  the  better,  but  even  quite  steeply  inclined  strata  may  yield 
good  productions.  Many  of  the  great  oilwells  in  Kussia  are 
drilled  into  strata  which  crop  out  at  the  surface  at  no  great 
distance :  the  new  and  much  boomed  field  of  Maikop  is  shown 
by  the  published  geological  maps  to  be  in  a  outcropping  series. 
In  Peru,  Trinidad,  and  some  parts  of  California  and  Mexico, 
good  productions  have  been  obtained  from  beds  that  crop  out 
in  monoclines.  It  is  to  be  noted  that  in  these  cases  the  oil  is 
asphaltic  and  of  fairly  high  specific  gravity,  the  latter  quality 
being  due,  partially  at  least,  to  inspissation. 

In  Burma,  where  a  light  oil  of  paraffin  base  is  the  character- 
istic petroleum,  no  adequate  production  has  ever  been  obtained 
by  drilling  in  a  monocline  to  strike  an  outcropping  oilsand. 
Native  hand-dug  wells  have  occasionally  been  worked  at  a 
profit  for  a  short  time  in  such  structures,  and  many  trial  bores 
have  been  made  at  great  expense,  but  all  have  been  abandoned 
finally  as  unprofitable  propositions.  Except  where  the  strata 
are  for  the  most  part  impervious,  and  there  is  a  probability  of 
striking  some  isolated  lenticular  bed  of  oilrock,  there  is  little 
hope  of  obtaining  paying  productions  of  light  mobile  oil  by 
drilling  in  a  monocline  where  all  the  horizons  crop  out.  Thus, 
the  class  of  oil  to  be  obtained  must  be  considered  in  relation  to 
the  structure;  conditions  favourable  for  an  asphaltic  oil  may 
be  quite  unfavourable  for  a  light  paraffin  oil. 

It  must  not  be  forgotten  in  dealing  with  a  monocline  that 
it  is  really  part  of  a  great  curve,  on  the  flank  of  either  some 
great  anticline  or  syncline.  Dips  do  not  continue  far  at  the  same 
angle  when  traced  downwards,  as  the  geologist  will  discover 


GEOLOGICAL   STRUCTURE  75 

at  once  in  plotting  sections.  Any  sudden  change  in  angle 
of  dip  may  have  great  effect  on  production.  Thus,  though  a 
well  may  begin  in  strata  dipping  at  45  degrees  or  more,  by 
the  time  the  oil-bearing  rocks  are  reached  the  dip  may  have 
decreased  to  20  degrees  or  less,  or  may  have  increased  and  even 
become  vertical.  Each  case  must  be  worked  out  from  the 
geological  map  of  the  area,  for  it  is  generally  quite  absurd  to 
calculate  on  a  dip  remaining  constant  for  any  considerable 
distance.  Before  any  wells  had  been  commenced  in  an  area  now 
being  exploited  by  a  company  in  Trinidad,  the  writer  (then 
Government  Geologist  in  that  colony)  worked  out  the  vertical 
and  lateral  changes  of  dip,  where  direct  evidence  of  the  inclina- 
tion of  the  beds  was  very  scanty  and  often  unreliable,  and 
furnished  those  responsible  for  the  exploitation  of  the  ground 
with  particulars  of  the  depth  to  the  observed  oil-bearing  bands 
in  different  places  and  the  angles  of  dip  at  which  each  would 
be  struck,  particulars  which  in  the  end  were  proved  to  come 
within  a  very  small  fractional  error  of  the  actual  results 
obtained.  In  this  case  projection  of  the  observed  dips  from 
the  nearest  reliable  section  would  have  given  an  entirely 
erroneous  result,  and  would  have  made  the  area  appear  very 
unfavourable  for  development  work.  Every  bit  of  evidence 
bearing  upon  change  of  dip  must  therefore  be  noted,  and  when 
outcrops  can  be  traced,  even  though  no  actual  observation  of 
dip  can  be  made,  it  is  often  possible  to  estimate  change  of  dip 
by  measuring  the  distances  between  two  known  outcrops.  The 
writer  has  often  found  this  method  of  great  service  in  obscure 
ground. 

Where  the  dip  in  a  monocline  suddenly  becomes  shallower, 
or  where  a  sudden  change  of  strike  occurs,  especially  where  a 
bend  in  the  strike  concave  to  the  direction  of  dip  in  the 
monocline  is  observed,  there  is  nearly  always  some  favourable 
effect  upon  the  concentration  of  petroleum.  The  oil  nearly 
always  is  found  to  have  migrated  towards  such  structures. 
This  can  be  readily  understood  in  the  case  of  a  bay  or  bend  in 
the  strike  which  is  frequently  accompanied  by  a  lowering  of  the 
dip ;  it  is  in  fact  an  abortive  anticline,  since  a  tilting  back  of 
the  monocline  to  a  position  of  horizontality  would  make  an 
anticline  or  dome  of  such  a  structure.  Instances  of  this  may 
be  studied  in  Trinidad,  perhaps  the  most  remarkable  being  at 
Pala  Seco  near  the  southern  coast  on  the  northern  flank  of  the 


76  OIL-FINDING 

great  southern  anticline.  Structures  such  as  this  may  be  due 
to  movements  earlier  than  that  responsible  for  the  monocline, 
and  the  concentration  of  petroleum  may  have  taken  place  prior 
to  the  great  movement  which  caused  the  main  flexures  of  the 
country.  However  this  may  be,  such  structures,  bays  in  the 
strike  concave  to  the  direction  of  dip,  always  favour  migration 
and  concentration  of  oil. 

Every  structure  produced  by  folding  can  be  classed  under 
one  or  other  of  the  heads  detailed  above. 

Faulting. — Faulted  areas  are  not  to  be  regarded  as  a  special 
class  of  structure,  but  faults  may  be  of  great  importance  struc- 
turally, so  that  it  is  necessary  to  give  some  account  of  how  they 
occur  in  oilfields  and  what  effects  may  be  attributed  to  them. 

The  flank  of  any  fold  may  be  replaced  by  a  fault,  partially 
or  wholly ;  in  this  case  it  is  a  strike  dislocation  having  the 
same  effect  as  a  very  sharp  unbroken  fold  would  have,  and 
obviously  due  to  the  same  movement  that  flexured  the  strata. 
In  many  cases  what  is  mapped  as  a  fault  at  the  surface  may 
become  a  sharp  flexure  at  some  distance  beneath,  where  the 
load  must  have  been  greater  during  the  period  of  movement, 
and  consequently  a  higher  coefficient  of  elasticity  of  the  strata 
can  be  postulated.  This  applies  both  to  "  normal "  and 
"  reversed  "  faults,  the  former  being  in  evidence  on  the  flanks 
of  symmetrical  or  gentle  flexures,  while  the  latter  are  frequent 
on  the  steeper  side  of  asymmetrical  folds  especially  where 
vertical  or  inverted  strata  are  observed. 

In  areas  under  insufficient  load  the  cohesion  of  the  strata 
may  be  overcome  under  flexuring  stresses,  and  faults  may  be 
developed  in  many  directions,  all,  however,  having  some  relation 
to  the  flexure  that  the  stresses  are  tending  to  form.  Indeed, 
it  is  only  under  such  conditions  that  what  is  called,  rather 
unfortunately,  a  "  normal "  fault  can  be  produced  at  all.  The 
"  normal  fault  "  of  the  textbooks,  a  dislocation  with  a  vertical 
or  nearly  vertical  displacement,  the  downthrow  being  in  the 
direction  of  hade,  is  by  no  means  a  common  phenomenon  in 
nature ;  it  is  only  under  simple  conditions  that  such  dislocations 
are  physically  possible. 

A  dislocation  must  begin  somewhere  and  must  die  out 
somewhere,  so  it  can  be  regarded  as  a  sag  or  tilt,  and  strike 
faults  whether  normal  or  reversed,  whether  thrusts,  "slides," 
or  the  doubtfully  possible  "  lags  "  of  some  authors,  are  direct 


GEOLOGICAL  STRUCTURE  77 

and  inevitable  special  phases  or  flexuring  movements.  Dip 
faults  on  the  other  hand  may  be  simple  tilts  or  sags,  or  may 
have  a  greater  or  less  horizontal  component.  Many  dip  faults 
which  map  as  normal  faults,  and  are  considered  as  such,  can 
be  proved  to  be  largely  horizontal  displacements,  thus  approxi- 
mating to  the  nature  of  "  wrench  faults."  Every  fault  must  be 
considered  in  relation  to  the  flexuring  movement,  and  thus  the 
observation  of  a  fault  should  be  a  help  rather  than  a  hindrance 
to  the  elucidation  of  structure,  since,  when  once  the  direction 
of  throw  has  been  determined,  it  shows  at  a  glance  what 
tendencies  of  movement  were  induced  in  the  strata  in  that 
particular  locality  by  the  stresses  to  which  they  were  subjected. 
A  map  may  be  complicated  by  faults,  but  the  structure  should 
be  explained  by  them  rather  than  rendered  more  difficult  to 
understand.  The  geologist  who,  after  describing  the  structure 
of  an  area,  concludes  by  saying  that  "  there  seems  to  be  a  good 
deal  of  faulting,"  or  who  tries  to  safeguard  his  views  of  a 
structure  by  saying  "  there  may  be  a  fault/'  which,  if  present, 
will  put  a  different  construction  on  the  evidence,  admits  by  so 
doing  that  he  has  not  mapped  the  area,  and  has  only  the 
vaguest  general  idea  of  its  geological  structure. 

Of  course  in  strata  of  Palaeozoic  or  Mesozoic  age  faults 
may  be  very  numerous  and  of  many  different  ages,  but  we  are 
dealing  principally  with  Tertiary  rocks  where  the  flexuring 
and  faulting  are  usually  simple,  and  the  stresses  that  caused 
them  easily  understood.  Petroleum  is  very  seldom,  if  ever, 
found  in  highly  faulted  and  contorted  strata  of  Palaeozoic 
age. 

Since  faults  and  folds  are  parts  of  the  same  earth-movement, 
the  effects  of  faults  upon  an  oilfield  need  not  necessarily  be 
prejudicial ;  strike  faults  may  indeed  help  to  ensure  a  greater 
concentration  of  the  petroleum  towards  the  crest  of  a  flexure, 
and  dip  faults  in  a  series  where  there  are  many  oil  sands  may 
bring  about  communication  between  different  sands,  and  so 
have  a  notable  local  effect  upon  production.  Where  the  bulk 
of  the  strata  are  impervious,  an  oilsand  which  would  otherwise 
crop  out  at  the  surface  may  be  cut  off  by  a  fault  and  sealed 
beneath  impervious  beds  (Fig.  2),  and  thus  yield  oil  under  much 
higher  pressure  when  pierced  by  the  drill  than  if  it  cropped  out 
in  the  vicinity. 

To  illustrate  the  interdependence  of  folding  and  faulting, 


78  OIL-FINDING 

and  at  the  same  time  the  effects  of  two  folding  movements 
in  different  directions,  we  may  take  the  Yedwet  inlier  in  the 
Magwe  district  of  Upper  Burma.  This  was  the  first  area 


FIG.  2. — Fault  sealing  up  an  oil-rush.     1.  Oilrock;  2.  Impervious 

strata. 

examined  by  the  writer  in  Burma,  and  it  proved  a  very 
fortunate  one  on  account  of  the  importance  of  the  evidence 
obtained. 

The  area  consists  of  an  inlier  of  the  Pegu  Series,  surrounded 
by,  and  in  places  capped  by  outliers  of,  the  uncon  form  able 
fluviatile  Irrawaddy  Series.  The  inlier  has  an  oval  outline, 
and  dips  are  gentle  throughout,  seldom  rising  to  more  than 
20  degrees,  and  that  only  towards  the  margins.  Presumably, 
then,  the  structure  was  dome-like.  Careful  examination  proved 
that  there  was  evidence  of  two  flexuring  movements,  both  very 
gentle,  one  tending  to  produce  flexures  running  E.  20  degrees  N. 
to  W.  20  degrees  S.,  of  which  two  were  recognized  in  the  area, 
and  another  tending  to  produce  flexures  running  almost  north 
and  south.  The  general  form  of  the  inlier  is  due  to  this  latter 
movement,  which  was  easily  identified  with  the  main  flexuriug 
movement  of  Burma.  The  presumption  was  that  the  former 
movement  was  an  earlier  one,  which  had  not  been  recognized 
previously  in  Burma. 

The  flexures  are  so  gentle  that  a  very  simple  case  of  the 
dynamic  conditions  produced  by  one  movement  on  the  results 
of  an  earlier  one  is  presented.  The  strata  had  evidently  not 
been  under  great  load  at  the  time  of  the  last  movement,  as 
a  number  of  small  faults  were  detected  in  various  parts  of  the 
area.  These  faults  are  of  the  same  age,  they  run  into  each 
other,  and  no  fault  displaces  another.  They  have  therefore 


GEOLOGICAL   STRUCTURE 


79 


evidently  been  caused  by  the  same  movement.  There  are  two 
main  directions  for  the  faults,  and  though  there  are  local 
modifications  and  variations,  the  lines  along  which  dislocation 
has  taken  place  are  wonderfully  constant  throughout  the  area, 
viz.  E.  20  degrees  N".  to  W.  20  degrees  S.  and  roughly  north  and 
south.  That  is  to  say,  the  systems  of  faults  are  parallel  to  the 
strike-lines  produced  by  the  two  movements. 

The  process  which  caused  these  faults  can  be  expressed  very 
simply  by  a  diagram  (Fig.  3).     Assuming  that  the  movement 


H 

FIG.  3.— E  F  =  crest  of  earlier  flexure ;  G  H  =  crest  of  later 
flexure  ;  h  —  faults  showing  down-throw. 

producing  the  two  flexures  running  E.  20  degrees  N.  to  W.  20 
degrees  S.  is  the  earlier,  we  have  a  force  AB,  impinging  obliquely 
upon  a  flexure  EF.  The  force  will  be  resolved  into  two  com- 
ponents AC  and  AD,  one  tending  to  increase  the  height  of  the 
fold  and  the  other  tending  to  compress  it  and  to  raise  a  flexure 
in  the  direction  BD.  It  is  a  simple  parallelogram  of  forces.  Let 


8o  OIL-FINDING 

us  consider  how  different  parts  of  the  area  will  be  affected.  A 
point  upon  the  crest  of  one  of  the  cross  flexures  will  tend  to 
rise,  especially  where  the  effects  of  both  movements  coincide. 
On  the  other  hand  a  point  in  the  syncline  between  the  two 
cross  flexures  will  be  affected  differently  according  to  its 
position  with  regard  to  the  second  movement.  Towards  the 
margins  of  the  area  a  point  in  these  synclines  will  tend  to  sink, 
towards  the  centre  of  the  area  one  component  will  tend  to  make 
it  rise  and  another  to  make  it  sink.  The  strata  in  such  localities 
will  be  under  a  peculiar  condition  of  stress,  and  adjustment 
will  be  arrived  at  by  the  developments  of  small  faults.  Thus 
we  get,  so  to  speak,  strike  faults  of  both  movements,  though 
produced  simultaneously  and  the  directions  of  throw  will  be  as 
shown  in  the  diagram.  All  these  faults  were  noted  and  mapped 
before  any  theoretical  ideas  as  to  their  origin  were  conceived. 
Keplacing  the  faults  by  folds  the  action  is  quite  easily  intelligible, 
and  can  be  reproduced  experimentally. 

The  evidence  obtained  from  this  area  was  applied  to  other 
fields  in  Burma,  and  served  to  explain  the  presence  of  faults 
in  many  localities  where  physical  reasons  for  their  origin  had 
not  been  ascertained.  One  of  the  first  results  was  the  proof  of 
the  relative  ages  of  the  two  movements ;  that  responsible  for 
the  cross  flexures  was  found  not  to  affect  the  younger  Irrawaddy 
Series,  while  the  other  movement  throws  it  into  great  folds. 
The  cross  movement  is  therefore  the  earlier. 

The  formation  of  dome  structures,  which  are  common  in 
inliers  of  the  Pegu  Series,  was  also  accounted  for.  It  is  almost 
entirely  due  to  elevation  by  the  earlier  movement  which, 
though  always  gentle,  has  had  the  effect  of  raising  parts  of  the 
series  locally  before  the  commencement  of  the  great  movement 
which  has  produced  the  main  strike-lines  of  the  country. 

Another  interesting  point  is  that  petroleum  has  never  been 
obtained  in  paying  quantity  in  any  field  that  does  not  show 
some  traces  of  the  earlier  movement,  even  though  these  traces 
are  often  almost  obliterated  by  the  much  more  powerful  later 
movement.  It  would  seem  that  there  has  been  a  preliminary 
concentration  of  the  petroleum  contents  of  the  strata  towards 
the  earlier  flexures,  which  concentration  has  been  greatly 
increased  afterwards  by  the  later  and  greater  flexuring. 

It  is,  of  course,  only  in  simple  cases  that  such  conclusive 
results  can  be  obtained  with  certainty,  but  the  Yedwet  inlier 


GEOLOGICAL   STRUCTURE  81 

will  serve  as  an  example  of  how  the  evidence  from  one  area 
may  assist  in  elucidating  the  more  complex  structures  of  other 
areas,  and  of  the  necessity  for  considering  flexures  and  faults 
together  and  not  separately. 

Unconformabilities. — The  only  other  phenomena  of  import- 
ance that  must  be  considered  when  dealing  with  the  geological 
structure  of  a  country  are  those  associated  with  unconforma- 
bilities.  Unconf  ormable  junctions  of  different  series,  or  of 
different  parts  of  the  same  series,  may  often  be  the  cause 
of  considerable  difficulty  in  the  working  out  of  the  structure 
of  an  oil-bearing  territory.  Among  Palaeozoic  or  Mesozoic 
rocks  there  may  be  no  difficulty  in  recognizing  an  uncon- 
formity, but  in  soft  or  lightly  compacted  Tertiary  strata  the 
discordance  may  never  be  seen  in  actual  section,  while  the 
rocks  both  above  and  below  may  have  very  similar  lithological 
characters,  and  fossil  evidence  may  be  wanting  or  too  scanty 
and  too  little  known  to  be  conclusive.  In  such  cases  there 
is  a  danger  of  an  unconf or  inability  being  unnoticed,  with 
disastrous  results  when  estimates  of  thickness  of  strata  and 
depth  to  oil-bearing  horizons  are  being  calculated. 

When  proved,  however,  by  careful  mapping,  an  unconforrna- 
bility  may  be  of  very  great  assistance  to  the  field  student  in 
giving  evidence  as  to  the  age  and  nature  of  the  earth-movements 
in  the  country  that  is  being  examined.  A  case  has  just  been 
quoted  from  Burma,  where  the  proof  of  the  relative  ages  of 
two  movements  depended  on  the  evidence  from  strata  of  two 
different  series,  separated  by  an  unconf ormability.  Had  the 
unconformability  between  the  Pegu  and  Trrawaddy  Series  not 
been  ascertained,  this  valuable  evidence  would  have  been  lost. 
The  discordance  between  these  two  series,  moreover,  is  some- 
times not  easily  recognized,  the  junction  has  several  times  been 
described  as  a  passage,  and  at  one  time  the  Geological  Survey 
of  India  were  actually  mapping  one  of  the  locally  basal  beds 
of  the  Irrawaddy  Series  as  the  topmost  bed  of  the  Pegu  Series. 

The  recognition,  therefore,  of  an  unconformity  becomes  a 
matter  of  very  great  importance,  and  it  must  be  distinguished 
from  a  plane  of  merely  local  or  contemporaneous  erosion. 
Local  erosion  is  very  frequent  among  rapidly  accumulated 
Tertiary  strata,  such  as  are  characteristic  of  areas  where  deltaic 
conditions  occurred  on  a  large  scale.  At  the  base  of  every 
thick  or  coarse-grained  arenaceous  group,  where  it  rests  upon 


82  OIL-FINDING 

finer  argillaceous  sediment,  there  are  nearly  always  some  signs 
of  erosion  of  the  underlying  strata,  and  the  divergence  in  strike 
and -dip  of  the  arenaceous  group  if  current-bedded  may  be 
considerable,  so  that  in  a  small  section  the  appearance  of  a  well- 
marked  unconformity  may  be  presented.  On  the  other  hand 
a  great  unconformability,  representing  a  gap  in  the  geological 
record,  may  appear  in  small  sections  to  be  a  perfectly  con- 
formable sequence. 

Careful  mapping  on  a  large  scale  will  always  make  certain 
of  any  unconformability  of  importance  by  disclosing  the  over- 
lap of  one  series  on  the  other,  but  where  evidence  is  very 
scanty,  or  when  there  is  not  sufficient  time  available  for 
detailed  work,  other  methods  must  be  relied  on. 

The  presence  of  some  mineral  or  minerals  or  fragments  of 
rock  in  one  series  and  not  in  the  other  is  a  bit  of  evidence  to 
be  noted  at  once,  as  it  points  to  the  strata  having  been  formed 
from  the  denudation  of  different  rocks,  so  that  if  any  such 
sudden  change  in  raineralogical  composition  is  detected  and 
proved  to  hold  good  over  any  considerable  distance  and  through 
thick  masses  of  strata,  a  prima  facie  case  for  the  presence  of 
an  unconformability  has  been  made  out. 

Differences  in  the  state  of  mineralization  of  the  strata  are 
also  to  be  noted,  the  bedding,  lamination  and  jointing  may  be 
of  different  characters  in  two  apparently  conformable  series, 
and  finally,  and  most  important  of  all,  the  lower  series  may 
show  evidence  of  small  folding  or  faulting  movements  that  do 
not  affect  the  upper  series. 

In  the  case  of  the  unconformability  between  the  Pegu  and 
Irrawaddy  Series  in  Upper  Burma,  the  point  was  established 
beyond  doubt  by  evidence  obtained  from  an  area  far  to  the 
northward  of  where  the  question  had  first  arisen  and  become 
of  importance.  The  Irrawaddy  Series  was  found  some  seventy 
miles  to  the  northward  to  contain  evidence  of  volcanic  action 
several  hundred  feet  above  its  base.  This  evidence  included 
outflows  of  lava,  and  formation  of  explosion-craters  with  beds  of 
ash  and  agglomerate.  The  ashes  contained  many  blocks  and 
fragments  of  metamorphic  rocks,  abundance  of  acid  lava  bombs 
and  fragments  including  beautifully  silicified  rhyolites.  The 
strata  of  the  Irrawaddy  Series  above  the  volcanic  beds  were 
found  to  contain  pebbles  of  metamorphic  rocks  and  agate,  and 
occasionally  much  decomposed  felspar  and  kaolin. 


GEOLOGICAL  STRUCTURE  83 

The  basal   beds   of  the   Irrawaddy  Series  in   the    Magwe 
District  to  the  southward,  where  the  unconformability  was  in 
question,  contain  well-rolled  pebbles  of  metamorphic  rock  and 
agate  (from  the  silicified  rhyolites),  while  kaolin  was  found  in 
some  of  the  sands.     None  of  these  appear  in  the  underlying 
Pegu  Series,  and  it  was  the  occurrence  of  kaolin  in  a  bed,  then 
considered  to  belong  to  the  Pegu  Series,  that  first  turned  the 
attention  of  the  writer  to   the  possibility  of  there  being  an 
unconformability.     Examination   of   intervening,    but   discon- 
tinuous, areas,  gave  confirmatory  evidence,  and  it  became  clear 
that  the  basal  beds  of  the  Irrawaddy  Series,  in  the  Magwe 
District  are  post-volcanic,  and  that  the  pre-volcanic  beds  of 
the   Irrawaddy   Series,   some   hundreds   of  feet   thick  in   the 
Pakokku  District,  have   either  never  been  deposited  in   the 
Magwe  District  or  have  been  removed  by  denudation.     Thus 
a  considerable  gap  in  the  succession  was  proved,  and  the  detec- 
tion of  pre- Irrawaddy  movements,  as  explained  above,  completed 
the  chain  of  evidence.     The  identification  of  fossil  horizons  in 
the  Pegu  Series  has  since  made  clear  that  this  unconformability 
is  often   of  very  great  extent,  and   that   great  thicknesses  of 
the   upper  beds   of  the   Pegu    Series   have  been  removed  by 
denudation  in   many  localities   before   the   deposition  of  the 
basal  beds  of  the  Irrawaddy  Series. 

Another  instance  of  unconformability,  for  long  a  matter  of 
doubt,  may  be  given  from  Burma,  namely,  the  unconformability 
between  the  basal  beds  of  the  Pegu  Series  and  the  underlying 
Bassein  Series,  probably  of  Eocene-Cretaceous  age.  This  line 
of  discordance  had  been  crossed  and  recrossed  by  several 
geologists  without  its  being  detected,  and  presumably  they 
classed  the  underlying  Bassein  Series  with  the  petroliferous 
Pegu  Series  above.  The  first  evidence  that  called  attention 
to  the  possibility  of  there  being  an  unconformity  was  afforded 
by  the  fact  that  the  great  littoral  arenaceous  group  of  the  Yaw 
sandstones,  which  forms  a  very  strong  feature  in  the  foothills 
of  the  Arakan  Yomas,  is  underlaid  by  softer  strata  with  a 
rather  different  aspect  as  regards  lamination,  bedding,  jointing 
and  state  of  mineralization.  Subsequently  evidence  of  move- 
ment, folds,  and  small  faults,  were  noted  in  the  underlying 
series  and  proved  not  to  affect  the  Yaw  sandstones,  and  finally 
the  mapping  on  the  six-inch  scale  of  a  few  square  miles, 
where  excellent  sections  can  be  seen,  proved  that  the  Yaw 


84  OIL-FINDING 

sand-stones  transgress  over  hundreds  of  feet  of  the  Bassein 
Series.  In  many  small  sections,  notwithstanding,  the  two  series 
differ  so  slightly  in  strike  and  dip  as  to  appear  perfectly 
conformable. 

In  Persia  the  detection  of  an  unconformability  proved  of 
the  greatest  importance  from  the  practical  point  of  view  of  oil- 
field development.  During  the  detailed  mapping  of  the  oilfield 
at  Maidan-i-Naphtun  the  possibility  of  there  being  such  an 
unconformability  had  been  suggested  by  the  discovery  of  the 
detrital  limestones  and  breccias,  and  the  occurrence  of  great 
conglomerates  at  various  horizons  in  the  Tertiary  series  full 
of  well-rounded  pebbles  of  limestone.  It  was  known  from  the 
work  of  previous  observers  that  a  great  mass  of  limestone  (the 
Asmari  limestone)  lay  at  a  lower  horizon,  and  it  had  been 
suggested  that  drilling  in  anticlines  of  this  limestone  might  be 
profitable.  When  the  Asmari  limestone  was  first  encountered 
by  the  writer  on  the  north-west  pitching  end  of  the  Asmari 
anticline,  evidence  of  unconformity  was  at  once  searched  for, 
but  beyond  thin  beds  of  detrital  limestone  resting  here  and 
there  on  a  somewhat  irregular  surface  of  the  calcareous  rock,  the 
occurrence  of  one  small  patch  of  breccia,  and  a  slight  discord- 
ance in  dip  and  strike  between  overlying  beds  of  gypsum  and 
the  Asmari  limestone,  no  evidence  was  forthcoming.  The 
various  kinds  of  limestone  preserved  in  pebbles  and  fragments 
in  the  conglomerates  and  breccias  near  Maidad-i-Naphtun  were, 
however,  matched  from  the  solid  outcrop  of  Asmari. 

A  few  days  later,  in  the  next  great  anticline  to  the  north- 
eastward, where  the  Asmari  limestone  again  appears,  demon- 
strative evidence  at  once  came  to  light.  A  great  transgression 
of  beds  high  up  in  the  oil-bearing  series,  chiefly  conglomerates 
full  of  limestone  pebbles,  was  observed  cutting  right  across  the 
anticline  of  limestone,  which  in  places  is  entirely  removed  by 
denudation,  some  2000  feet  of  thickness  having  been  denuded. 
On  the  flanks  of  the  limestone  outcrop  bed  after  bed  of  the  oil- 
bearing  series  makes  its  appearance  between  the  calcareous 
rock  and  the  great  conglomerates  which  lie  across  the  denuded 
anticline. 

Subsequent  field-work  proved  that  these  anticlines  were 
denuded  as  they  rose  under  the  flexuring  movement,  and 
that  the  succession  may  be  entirely  conformable  in  the  syn- 
clines;  consequently  at  the  north-west  end  of  Asmari  Hill, 


GEOLOGICAL   STRUCTURE  85 

where  the  fold  of  lime- stone  pitches  sharply  downwards,  no 
striking  evidence  of  unconformity  could  be  expected. 

The  point  of  importance  from  the  oil-development  point 
of  view  is  that  the  oil-bearing  strata  belong  to  a  different 
series,  and  are  of  later  age  than  the  Asmari  limestone,  and 
to  attempt  drilling  in  the  latter  would  be  entirely  speculative 
and  unjustifiable.  But  for  the  detection  of  this  unconformability 
we  should  be,  as  far  as  that  region  in  Persia  is  concerned,  still 
in  the  dark  as  regards  the  conditions  under  which  these 
Tertiary  strata  were  deposited,  and  as  to  what  districts  are 
most  favourable  for  exploitation. 

In  southern  Ohio  probable  unconformities  that  do  not  crop 
out  at  the  surface  have  been  detected  in  strata  either  horizontal 
or  very  gently  dipping,  and  the  transgression  is  sometimes 
exceedingly  regular.  Mr.  Frederick  G.  Clapp  has  explained 
this  matter  very  clearly  in  one  district,  showing  that  the 
Clifton  sand  (a  well-known  oil-bearing  horizon)  increases  in 
depth  eastward  at  a  rate  of  from  30  to  100  feet  per  mile  more 
than  is  indicated  by  the  datum  line  given  by  a  characteristic 
bed  exposed  on  the  surface.  In  such  a  case  the  evidence  from 
wells  becomes  more  important  than  a  detailed  study  of  the 
surface.  But  such  remarkable  regularity  must  be  exceedingly 
rare,  and  there  is  always  the  possibility  that  the  effect  may 
be  due  to  lateral  variation,  the  thinning  or  thickening  of  the 
oilsands  and  the  beds  intervening  between  them  and  the 
surface,  rather  than  to  unconformability  and  overlap.  Varia- 
tions in  thickness  as  great  as  this  when  a  series  is  traced 
far  in  the  same  direction  may  be  observed  in  the  Sabe  and 
Yenankyat  fields  in  Burma,  where  a  series  of  deep  valleys 
across  the  strike  make  it  possible  to  measure  the  thicknesses 
of  groups  in  actual  sections.  In  cases  such  as  that  of  Southern 
Ohio  the  evidence  from  wells  is  essential,  and  it  is  a  matter 
more  for  the  consideration  of  the  oil-engineer  than  for  the 
geologist,  whose  examination  of  the  evidence  at  the  surface 
should  be  complete  before  wells  are  drilled  in  a  new 
field. 

Unconformabilities,  the  extent  and  directions  of  increase 
of  which  have  not  yet  been  worked  out,  are  already  causing 
difficulties  to  those  entrusted  with  the  exploitation  of  some 
areas  in  Trinidad,  and  have  perhaps  done  something  to  con- 
firm the  popular  idea  that  petroleum  is  a  very  capricious 


86  OIL-FINDING 

mineral,  and  that,  as  the  driller  is  fond  of  reiterating,  "  the 
only  way  to  find  oil  is  to  drill  a  hole  for  it." 

But  of  all  oilfields  the  successful  development  of  which 
depends  on  a  study  of  unconfor inability  and  overlap,  and  the 
directions  in  which  erosion  of  the  denuded  surfaces  beneath 
unconformable  junctions  increase  or  decrease,  the  most  remark- 
able is  perhaps  the  island  of  Barbados.  Here  we  have  evidence 
of  folding  movements  of  considerable  severity  acting  in  different 
directions  and  at  different  times.  There  are  two  great  uncon- 
formities. The  oil-bearing  series,  much  folded  and  not  a  little 
faulted,  is  overlaid  unconformably  by  a  series  of  oceanic 
deposits,  which  in  their  turn  have  been  thrown  into  flexures, 
raised  within  the  zone  of  denudation,  and  overlaid  unconform- 
ably by  a  thick  mass  of  coral  limestone  of  comparatively 
recent  date,  which  rises  in  terrace  after  terrace  lying  hori- 
zontally and  covers  by  far  the  greater  part  of  the  island. 
The  surface  of  the  oil-bearing  series  is  irregular,  and  there 
is  an  overlap  of  the  upper  beds  of  the  Oceanic  series  over 
the  lower,  while  there  is  another  overlap  of  the  coral  lime- 
stone over  the  Oceanic  series,  which  has  doubtless  been 
removed  by  denudation  in  many  places,  so  that  the  coral 
limestone  rests  directly  on  the  petroliferous  series  (the  Scot- 
land Beds)  in  several  districts.  Yet  in  spite  of  sharp  folding, 
faulting  and  unconformabilities,  oil  has  been  produced  in  small 
quantities  for  several  years,  and  though  the  work  has  not  been 
a  great  commercial  success  up  to  date,  there  are  prospects  of 
valuable  oilfields  being  proved.  Success  will  depend  upon  the 
working  out  of  the  effects  of  the  different  movements  and  con- 
sequent unconformabilities,  and  the  determination  by  such 
methods  of  where  the  petroliferous  strata,  whether  deeply 
buried  beneath  younger  deposits  or  not,  will  be  found  under 
conditions  most  favourable  for  good  productions  of  oil. 

Sufficient  has  been  written  to  show  that  unconformabilities 
are  common  phenomena  in  Tertiary  oilfields,  and  that  they 
must  be  studied  carefully  if  the  structure  of  a  country  is  to 
be  ascertained  beyond  the  possibility  of  doubt.  They  are  of 
great  practical  importance  to  any  company  undertaking 
development  work. 

Thus  folds,  faults,  and  unconformabilities  must  be  con- 
sidered together  and  in  detail  before  any  connected  history 
of  a  country  or  district  can  be  presented,  and  it  may  often  be 


GEOLOGICAL  STRUCTURE  87 

necessary  to  visifc  areas  far  beyond  the  confines  of  a  district 
before  some  of  the  problems  in  structure  that  it  exhibits  can 
be  solved.  There  must  be  no  such  thing  as  opinion  about 
geological  structure;  only  the  facts  will  suffice,  and  the 
geologist  must  make  absolutely  sure  of  structure  if  the  drill- 
ing programme  is  to  be  directed  with  the  least  possible 
number  of  failures  and  the  greatest  number  of  successful 
results,  since  in  the  area  selected  through  knowledge  of  the 
oil-bearing  series  and  its  lateral  variations  it  is  the  geological 
structure  and  nothing  else  that  determines  the  extent  of  each 
field. 

An  oilfield  with  several  producing  wells,  but  with  no 
geological  map,  may  be  part  of  a  great  potential  producing 
area,  or  may  be  the  merest  fringe  in  which  oil  production 
is  possible.  The  area  between  two  producing  wells  is  not 
developed  or  proved,  unless  the  geological  structure  of  the 
intervening  ground  is  known,  and  known  to  be  favourable. 
But  a  very  few  wells,  carefully  located,  will  enable  the  geologist 
to  determine  within  reasonable  limits  the  probable  productive 
area  of  a  field. 

Hence  every  detail  of  dip,  strike,  change  in  dip  or  strike, 
hade  of  axes  of  flexures,  and  pitch  of  axial  lines  must  be 
noted,  and  if  the  area  be  undulating  the  height  of  each  locality 
where  observations  have  been  made  about  a  datum  line  should 
be  ascertained.  Then  and  only  then  can  absolute  certainty 
as  to  structure  be  achieved. 


CHAPTER  VI 
INDICATIONS   OF   PETROLEUM 

OUR  Manager  cables  as  follows  : — "  Borehole  No.  3  has  reached 
a  depth  of  792  feet,  and  the  indications  are  favourable."  To 
how  many  meetings  of  anxious  shareholders  have  such  or 
similar  comforting  words  been  read,  and  how  often  do  we  see 
a  message  of  this  nature  dealing  with  a  new  field  under 
exploitation  quoted  in  the  public  press  ?  And  it  would  be  a 
very  bold  and  even  impudent  shareholder  who  would  rise  in 
his  place  and  ask  pointedly :  "  What  are  the  indications,  and 
why  are  they  considered  favourable  ?  " 

Such  queries  would  no  doubt  receive  answers,  but  in  all 
probability  they  would  be  vague  and  carefully  guarded 
statements,  for  the  Chairman  or  Managing  Director  of  a 
Company  may  very  naturally  consider  that  it  is  not  his  duty 
to  study  geological  data ;  he  depends  upon  the  Manager  or 
Field-Superintendent,  who  has  cabled ;  or  the  log  of  the  well 
has  been  submitted  to  an  expert  at  home,  who  has  pronounced 
the  indications  "favourable."  And  the  shareholders  may  go 
away  satisfied,  though  it  may  be  that  neither  Field-Manager 
nor  expert  has  any  certain  knowledge  of  what  would  be 
"  favourable "  indications  in  the  locality  and  at  the  depth 
stated. 

This  at  once  raises  the  question  of  what  are  favourable 
indications  of  petroleum,  i.e.  indications  that  point  to  the 
probability  of  good  productions  being  obtained. 

The  subject  naturally  divides  itself  into  (1)  Surface 
Indications,  and  (2)  Indications  in  a  borehole. 

(1)  Surface  Indications. — It  is  to  indications  at  the  surface 
that  attention  has  always  been  attracted.  The  expert  who 
visits  a  new  district  goes  first  to  the  localities  where  "  shows," 
as  they  are  called,  are  to  be  seen,  and  it  is  largely  by  the 
presence  of  "  shows  "  in  any  piece  of  land  that  it  is  judged 

88 


INDICATIONS   OF   PETROLEUM  89 

by  persons  without  technical  knowledge.  The  field-student 
will  do  well  to  make  himself  acquainted  as  soon  as  possible 
with  the  nature  of  the  "  shows "  which  he  may  expect  to  find 
in  the  country  that  he  is  examining.  He  has,  let  us  say, 
made  his  preliminary  traverses,  gained  some  idea  of  the  lateral 
variation,  and  discovered  that  favourable  structures  produced 
by  the  earth-movements  he  has  been  studying  are  to  be  found. 
The  time  now  comes  for  him  to  study  the  indications  at  the 
surface  as  a  guide  to  what  thicknesses  of  strata  and  what 
horizons  may  be  expected  to  prove  petroliferous,  and  what 
variety  of  oil  is  present. 

Let  it  be  admitted  at  once  that  the  actual  shows  of  oil 
are  of  great  importance,  much  is  to  be  learnt  from  them; 
but  the  study  of  structure  must  take  first  place.  It  is  a 
surface  show  that  always  attracts  the  lay  mind.  During  the 
writer's  first  examination  of  an  oilfield  he  inadvertently 
grieved  an  enterprising  pioneer  who  had  pointed  out  a  small 
seepage  with  the  remark  "  there  is  what  would  make  glad  the 
heart  of  a  Bockefeller,"  by  bluntly  answering  that  he  himself 
took  little  interest  in  such  indications  as  long  as  the  geological 
structure  was  still  unsolved.  As  a  matter  of  fact  it  is  very 
frequently  where  surface  shows  of  oil  are  seen  that  drilling 
would  be  entirely  unsuccessful,  and  many  of  the  greatest 
oilfields  known  to-day  have  not  a  single  surface  indication 
within  their  length  and  breadth. 

Surface  indications  are  of  various  kinds  according  to  the 
class  of  oil,  the  nature  of  the  strata,  and  the  geological 
structure.  They  comprise  : — 

(a)  Seepages  of  oil. 

(b)  Asphalt  deposits. 

(c)  Evolution  of  gas  from  gas-pools,  mud-volcanoes  or 

dry  ground. 

(d)  Outcrops  of  bituminous  strata, 

and 

(e)  Veins  of  manjak  or  ozokerite. 

In  addition  to  these  the  evolution  of  hydrogen  sulphide 
may  be  in  some  cases  a  favourable  indication,  and  crystals  of 
sulphur  in  cavities  in  a  rock,  or  the  presence  of  minute  traces 
of  sulphur  in  flecks  and  patches  may  also  be  important.  Belts 
of  stunted  or  sickly  vegetation  may  give  a  valuable  indication 


90  OIL-FINDING 

where  no  solid  evidence  is  available.  Finally  a  faint  odour 
ef  petroleum  may  sometimes  be  detected  where  no  actual 
seepage  can  be  discovered. 

(a)  Seepages  of  Oil. — Where  an  oilrock  reaches  the  surface 
there  is  generally  some  sign  of  petroleum.  It  should  be  looked 
for  in  low  ground,  in  the  beds  of  streams,  or  at  the  foot  of 
hills,  and,  if  the  strata  be  bent  into  anticlinal  form,  at  or  near 
the  crest  of  the  anticline.  In  many  cases  where  the  upper 
part  of  an  outcrop  has  lost  all  signs  of  petroleum  through 
weathering,  a  seepage  will  be  noticed  where  the  outcrop  crosses 
the  valley  of  some  small  stream  or  gully.  In  such  localities 
films  of  oil  with  a  beautiful  iridescence  may  be  seen  on  the 
surface  of  the  water.  The  odour  will  at  once  distinguish  these 
films  from  decomposing  bicarbonate  of  iron  which  also  gives 
an  iridescent  film  (of  hydroxide),  and  which  has  often  been 
mistaken  for  evidence  of  petroleum.  The  films  in  these  two 
cases,  however,  are  by  no  means  identical,  and  when  seen  side 
by  side  could  never  be  mistaken. 

If  the  seepage  be  more  copious,  brown  or  greenish  or  black 
drops  of  oil  may  be  seen,  and  these  may  collect  into  patches 
on  the  water  near  their  source  or  in  eddies  and  still  pools 
down  stream.  Gas  is  frequently  seen  bubbling  up  through 
the  water.  In  some  cases  actual  trickles  of  oil  out  of  the 
rock  may  be  observed.  But  the  greater  part  of  the  outcrop 
of  an  oilrock  will  probably  give  no  indication  of  being 
petroliferous  until  dug  into  for  a  few  inches  or  perhaps 
feet. 

The  cavernous  detrital  limestones  of  Maidan-i-Naphtun 
exude  oil  rapidly  in  the  valleys  of  streams,  and  where  the 
water  is  clear  small  spherical  drops  of  the  oil  may  be  seen 
emerging  from  cavities  and  rising  to  the  surface.  But  the 
greater  part  of  the  outcrop  is  barren  of  indications.  The 
greatest  natural  show  of  liquid  petroleum  which  the  writer  has 
seen  occurs  in  this  field ;  as  much  as  20  barrels  a  day  of  oil 
flow  to  waste  in  one  stream.  Three  or  four  brisk  seepages 
combine  to  make  up  this  quantity,  and  from  time  immemorial 
the  Shusteris  have  collected  the  petroleum  and  burnt  off  the 
light  oils  to  obtain  bitumen. 

Another  remarkably  large  seepage  occurs  in  the  Trinity 
Hills  Forest  Eeserve  in  Trinidad.  At  the  time  of  the  author's 
visit  to  this  spot  a  stream  some  three  yards  in  breadth  was 


INDICATIONS    OF   PETROLEUM  91 

covered  entirely  with  a  dark  brown  oil  with  green  fluorescence 
for  a  distance  of  nearly  a  hundred  yards,  while  gas  bubbled  up 
briskly  both  through  the  water  and  from  several  places  on  the 
banks.  This  show  is  on  an  outcrop  of  the  Galeota  Oil-bearing 
Group. 

Oils  with  a  paraffin  base  usually  make  smaller  and  less 
striking  seepages  than  asphaltic  oils,  as  the  results  of  inspissa- 
tion  are  more  readily  washed  away  by  rains,  and  the  rock  from 
which  the  oil  exudes  is  more  easily  and  quickly  robbed  of  its 
petroleum  contents  under  weathering  processes.  Many  of  the 
outcrop  shows  in  Burma,  where  the  oil  is  generally  light  and 
full  of  solid  paraffin,  consist,  even  on  the  outcrops  of  thick  oil- 
sands,  of  very  small  pools  not  more  than  a  foot  or  two  in 
diameter  in  the  courses  of  small  ravines  and  stream  valleys. 

Oil  obtained  from  seepages  is  always  more  or  less  inspis- 
sated, and  does  not  give  a  fair  sample  of  what  may  be  obtained 
by  drilling,  the  light  fractions  having  evaporated. 

An  asphaltic  oil  can  usually  be  distinguished  from  a  paraffin- 
base  oil  by  the  manner  in  which  it  inspissates ;  the  former 
generally  remains  liquid  or  semi-liquid  for  a  longer  time  but 
dries  finally  to  black  asphalt ;  the  latter  soon  coagulates  into 
little  flakes  often  of  a  reddish  brown  colour,  and  when  present 
in  quantity  and  containing  much  solid  paraffin  dries  into  a  soft 
mass  like  vaseline,  which  does  not  adhere  to  exterior  objects 
with  the  same  tenacity  exhibited  by  asphaltic  oil,  and  is  con- 
sequently more  easily  washed  away. 

The  most  remarkable  seepages  of  oil  are  those  that  have 
been  naturally  filtered,  and  partly  or  entirely  decolorized.  In 
such  cases  the  petroleum,  though  it  has  probably  lost  its  most 
volatile  constituents  by  inspissation,  has  also  been  deprived  of 
the  bulk  of  its  heavier  fractions  by  filtration.  The  "  white  oil " 
of  Kala  Deribid  in  Persia  has  already  been  mentioned ;  it  is  a 
limpid  mobile  liquid  that  the  writer  could  hardly  believe  to  be 
oil  till  he  had  dipped  his  hand  in  it. 

Another  interesting  example  of  a  filtered  oil,  this  time  of 
asphaltic  base,  may  be  observed  exuding  from  an  outcrop  of 
sandy  clays  in  a  small  tributary  of  the  Lizard  Eiver  in  the 
south-eastern  corner  of  Trinidad.  The  locality  has  been  called 
"Lizard  Spring."  The  oil  is  dark  brown  with  a  green 
fluorescence,  and  it  collects  on  the  surface  of  the  water  in  the 
stream  bed.  When  the  writer  was  encamped  in  the  forests 


92  OIL-FINDING 

near  this  spot  a  sample  of  the  oil  was  skimmed  by  means  of  a 
leaf  from  the  surface  of  the  water,  bottled,  and  taken  into 
camp,  where  it  was  burnt  that  night  in  a  small  open  lamp, 
hardly  clogging  the  wick  at  all.  An  analysis  of  the  oil 
collected  in  this  manner  at  Lizard  Spring  was  made  some  years 
ago  by  Professor  Carmody,  Government  Analyst  of  Trinidad, 
who  found  it  distilled  like  a  refined  oil,  and  gave : — 

Petroleum  spirit  .....  0 

Illuminating  oil  .....  73 

Lubricating  oil 25 

Residual  bitumen 2 

100 

The  specific  gravity  was  '867,  and  the  flash  point  was  above  145 
degrees  (Abel's  test).  This  oil  is  sufficiently  inspissated  to  make 
it  a  perfectly  safe  burning  oil,  and  to  contain  a  fair  percentage 
of  heavy  oil  and  residue.  A  year  or  two  later  a  small  excava- 
tion was  made  in  the  outcrop,  at  the  author's  suggestion,  to 
obtain  a  fresher  sample  of  the  oil.  Professor  Carmody's 
analysis  showed  this  second  sample  to  contain : — 

Petroleum  spirit  .         .         .         .         .12 
Illuminating  oil    .         .         .        -.         .        81*25 
Lubricating  oil      .....          6 
Residual  bitumen          .         .         .  0*75 

few 

The  specific  gravity  was  considerably  lower.  This  is  obviously 
a  dangerous  oil  on  account  of  its  percentage  of  petroleum  spirit, 
and  could  not  be  burnt  with  safety  in  a  lamp.  The  two 
analyses  are  interesting  as  showing  the  effects  of  inspissation. 
The  oils  are  both  well  filtered  by  passage  through  the 
argillaceous  strata,  and  it  is  hardly  necessary  to  say  that  were 
a  borehole  drilled  at  this  spot  an  oil  of  this  class  would  not  be 
obtained  in  any  quantity,  though  heavier  unfiltered  oils  from 
the  same  source  would  probably  be  struck. 

Evolution  of  oil  is  not  unfreqently  observed  in  the  sea, 
where  an  oil-bearing  stratum  is  exposed  beneath  the  water. 
In  the  Caspian  Sea  such  shows  were  well  known  for  many  years 
before  any  active  drilling  was  undertaken  at  Baku. 

Off  the  coast  of  Trinidad  there  are  many  places  where  oil  is 
occasionally  to  be  seen.  Perhaps  the  best  known  is  just  west 


INDICATIONS    OF   PETROLEUM  93 

of  the  famous  Pitch  Lake,  where  a  brisk  evolution  of  gas  with 
drops  of  brown  oil  may  be  observed  about  a  quarter  of  a  mile 
off  shore.  The  activity  of  this  show  varies  considerably,  but  on 
a  breezy  day  the  locality  can  usually  be  detected  by  the 
presence  of  a  patch  of  smooth  water,  the  film  of  oil  covering  it 
being  sufficient  to  prevent  waves  from  breaking. 

At  the  mouth  of  the  Vance  River,  and  again  at  Point 
Ligoure,  where  outcrops  of  the  Rio  Blanco  Oil-bearing  Group 
run  out  to  sea,  the  water  is  sometimes  covered  with  a  film  of 
oil  for  a  considerable  distance.  Other  submarine  shows  near 
the  eastern  and  south-eastern  coasts  are  sporadic  and  occasion- 
ally of  explosive  violence ;  after  an  outburst  sticky  oil  and  soft 
asphalt  are  washed  up  on  the  shore  in  considerable  quantity. 

Off  the  south-western  corner  of  Tobago  there  is  apparently 
a  submarine  outcrop  of  oilrock,  for  sticky  inspissated  petroleum 
is  washed  up  on  the  beach  and  the  coral  limestone  in  great 
quantity  at  some  periods  of  the  year. 

(b)  Asphalt  Deposits. — Oils  of  asphaltic  base  nearly  always 
make  their  presence  obvious,  when  the  conditions  are  favour- 
able, by  more  or  less  extensive  deposits  of  asphalt  along  the 
outcrops  of  the  petroliferous  strata.  There  is,  of  course,  no 
hard-and-fast  line  between  a  seepage  of  crude  oil  and  a  deposit 
of  asphalt;  every  gradation  of  sticky  and  inspissating  oil 
between  the  two  may  be  observed  on  the  same  outcrop.  In 
Trinidad,  where  most,  though  not  all,  of  the  oils  are  asphaltic, 
the  phenomena  of  asphalt  deposits  can  be  studied  on  a  remark- 
able scale.  Foremost  of  all  comes  the  famous  Pitch  Lake,  the 
best  known,  though  not  the  most  extensive,  asphalt  deposit  in 
the  world.  Much  has  been  written  about  it,  and  many  theories 
have  jDeen  propounded  to  account  for  the  origin  of  this  lake. 
Without  going  in  detail  through  the  theories  of  various  authors 
and  pointing  out  where  each  has  advanced  the  knowledge  of 
the  day,  it  may  be  as  well  to  give  a  brief  description  of  the 
field  evidence  and  the  last  published,  and  so  far  accepted, 
theory;  the  author  may  be  pardoned  for  inserting  a  lengthy 
quotation  from  his  official  account  from  the  Council  Papers  of 
Trinidad,  Ho.  60  of  1907,  more  particularly  as  this  account  has 
been  drawn  upon  extensively  by  others,  and  large  portions  of 
it  published  verbatim  and  without  acknowledgment. 

"  A  brief  account  of  the  evidence  obtained  in  the  field,  and 
from  other  sources,  must  be  given.  The  Pitch  Lake  lies  upon 


94  OIL-FINDING 

a  well-defined  plateau  138  feet  above  sea  level.  The  area  has 
recently  been  affected  by  gradual  upheaval,  as  proved  by  raised 
beaches  in  the  neighbourhood,  and  it  is  probable  that  the 
plateau  at  no  distant  date,  geologically  speaking,  stood  at  or 
below  sea  level,  and  is  in  fact  a  raised  beach  or  coastal  beach 
itself. 

The  geological  structure  is  a  gentle  anticline  which  runs 
roughly  east  and  west,  the  lake  being  upon  the  crest.  The 
vicinity  of  the  lake  is  almost  entirely  covered  with  surface 
deposits  concealing  the  solid  evidence.  The  underlying  rocks 
are  lightly  compacted  and  are  often  disintegrated  to  a  great 
depth,  and  the  surface  wash  of  disintegrated  material  covers 
almost  all  the  ground.  The  "  brown  shales "  mentioned  by 
Messrs.  Louis  and  Gordon,  though  often  giving  an  appearance 
of  stratification,  are  not  Tertiary  sediments,  but  recent  surface 
deposits.  The  brown  colour  is  due  to  the  presence  of  finely 
divided  bitumen  or  asphalt  dust. 

The  La  Brea  oilsand,  a  deposit  of  variable  thickness,  is  the 
source  of  all  the  pitch.  It  crops  out  to  westward  of  the  lake  in 
the  coast  section,  to  eastward  of  the  plateau,  and  also  to  the 
southward  near  the  Vessiny  Eiver,  and  in  inliers  in  hollows. 
Its  outcrop  has  been  mapped  for  several  miles.  This  oilrock  is 
covered  by  a  fine  bluish  clay,  which,  when  impregnated  suffi- 
ciently with  bituminous  material,  has  occasionally  become 
ignited  and  burnt  to  porcellanite,  e.g.  south  and  south-west  of 
the  lake.  The  clay  in  its  turn  is  covered  by  a  soft  yellow 
sand,  the  disintegrated  outcrop  of  which  covers  much  of  the 
area  north  of  the  lake. 

Wherever  the  capping  of  clay  is  thin,  or  the  oilrock  is 
merely  covered  by  superficial  deposit,  or  is  actually  exposed, 
soft  asphalt  exudes,  forming  small  cones,  examples  of  which 
may  be  seen  beside  the  road  between  the  Asphalt  Company's 
works  and  the  lake,  and  at  several  places  north  and  west  of 
the  lake. 

The  oilrock,  where  it  is  exposed  on  the  shore  west*  of  the 
lake,  is  a  fine  dark  sand,  so  full  of  bitumen  that  the  superficial 
layers  actually  flow  slowly,  the  semi-liquid  asphalt  as  it  exudes 
carrying  the  inorganic  material  of  the  rock  with  it.  Pieces  of 
this  rock  may  be  twisted  off  in  the  fingers  and  rolled  into 
pellets.  An  analysis  of  a  specimen  by  the  Government  Analyst 
gives  the  following  results  : — 


INDICATIONS    OF   PETROLEUM  95 

Water,  etc.,  volatile  at  100°  C. 
Bitumen         .         .         **       .        ,  •    .. 
Non-bituminous  organic  matter      .      .  . 
Ash 


Soluble  in  petroleum  ether    .         *         .     8  per  cent. 
This  specimen  was  taken  from  a   weathered   tide-washed 
outcrop.     The  quantity  of  non-bituminous  organic  matter  is 
remarkable,   but,  as  will  be  seen  later,  recent  work  by  Mr. 
Clifford  Eichardson  has  thrown  much  light  upon  this  point. 

A  shallow  boring  (about  60  feet)  was  made  in  the  outcrop 
of  oilrock  west  of  the  lake,  many  years  ago.     It  is  situated  200 
feet  from  the  sea  and  yields  a  small  quantity  of  rather  heavy 
oil.   A  sample  taken  from  the  surface  gave  the  following  results 
on  analysis  by  the  Government  Analyst : — 

Specific  gravity  .      .  .        .         .         .       0*950 

Mineral  matter .         .         .         .  0'02    per  cent. 

On  distillation — 

Water        ./       .         .         .     "  .  •     V      1-2 
Petroleum  spirit          ,  -      .  •     .         .     12*8 
Illuminating  oil  (150°  -  300°  C.)        .     36'0 
Lubricating  oil  (above  300°  C.)  .         .     32'0 
Eesidual  bitumen        .  •    ,  .  •       .  •       .     12*3 
Loss  .        .         .        .        .       ".        .       5*7 

100-0 

In  the  sea  at  a  distance  of  about  200  yards  west  south-west 
of  the  last-mentioned  locality,  there  is  an  oilspring.  A  smooth 
patch  on  the  water  is  often  conspicuous,  and  in  it  drops  of 
brown  oil  may  be  seen  floating,  while  gas  bubbles  up  all  round, 
and  a  film  of  oil  sufficient  to  prevent  waves  from  breaking 
sometimes  covers  the  surface  for  a  considerable  distance. 

In  the  hollow  east  of  the  plateau  on  which  the  lake  is 
situated,  the  oilrock  crops  out  again,  and  large  flattened  cones 
of  semi-liquid  asphalt  may  be  seen  with  slight  evolution  of  gas. 
In  these  cones  or  rather  pools  of  soft  pitch  the  material  can  be 
seen  exuding,  and  it  is  streaky  with  the  quantity  of  inorganic 
matter  brought  up  with  the  bitumen,  indicating  that  either  the 
cohesion  of  the  oilrock  breaks  down  when  it  is  exposed,  or  that 
superincumbent  material  is  carried  up  by  the  flow  of  asphalt 
and  gradually  absorbed  in  it. 


96  OIL-FINDING 

Borings  made  by  the  Asphalt  Company  in  1893  have 
furnished  additional  evidence  of  the  underlying  oilrock.  In 
the  centre  of  the  lake  a  depth  of  135  feet  was  reached  without 
touching  bottom,  but  at  1000  feet  from  the  centre  on  the  north 
side  fine  sand  was  struck  at  80  feet,  then  more  asphalt,  and 
at  90  feet  asphaltic  sand,  i.e.  the  more  or  less  disintegrated 
oilrock.  A  boring  south  of  the  lake  also  struck  a  hard 
asphaltic  sand,  obviously  the  same  which  crops  out  to  the 
east-south-east,  the  course  of  which  can  be  traced  by  lines  of 
asphalt  cones.  The  oilrock  cannot  be  identified  in  the  coast 
section  in  Guapo  Bay,  but  porcellanite  and  lignitic  shales 
covered  by  sands  and  sandy  clays  probably  represent  it,  and 
indicate  that  the  oilrock  is  thinning  out  and  the  oil-producing 
conditions  at  this  horizon  ceasing  in  this  direction. 

The  next  evidence  to  be  considered  is  the  composition  of 
the  lake  pitch.  This  is  treated  of  so  fully  in  Mr.  Clifford 
Eichardson's  book,  "The  Modem  Asphalt  Pavement,"  that  a 
few  brief  quotations  will  suffice.  The  average  composition  of 
the  lake  pitch  is  given  as : — 

Water  and  gas  .         .         .29  per  cent. 

Organic  matter,  not  bitumen         .         .       7 

Mineral  matter 25 

Bitumen 39 

100 

The  asphalt  is  an  "emulsion"  of  these  constituents.  The 
inorganic  matter  consists  of  fine  sand  and  clay  with  a  small 
quantity  of  iron  oxide  and  soluble  salts.  Mr.  Clifford  Eichardson 
gives  an  analysis  of  the  mineral  matter  as  follows  :— 

Si02  .....  70-64 

A12O3  ....  17-04 

Fe203  ;        i       .         .  7-62 

CaO  .  .  .  ;***      ;...      .  0'70 

MgO.  .«      <        .        .  0-90 

Na2O  •••.         .         .         .  1-56 

K20  .  ;         .         .         .  0-35 

S03   .  *  -      .        .        .  0-97 

Cl      .  ,  '      .         .         .  0-22 

100-0 


INDICATIONS   OF   PETROLEUM  97 

This  corresponds  with  the  composition  of  a  normal  sandstone, 
with  slight  admixture  of  argillaceous  material.  The  micro  - 
photograph  of  the  mineral  matter  which  Mr.  Clifford.  Eichardson 
published  ("  The  Modern  Asphalt  Pavement,"  p.  34)  shows  all 
the  characteristics  of  the  debris  from  an  ordinary  fine  water- 
borne  sandstone,  the  grains  not  being  greatly  abraded  as  in 
windblown  sands,  nor  having  any  of  the  characteristics  of  silica 
deposited  from  solution.  The  finest  material  is  a  fairly  pure 
clay.  The  percentage  of  "  Organic  matter  not  bitumen " 
presents  a  point  of  great  interest;  as  recorded  above,  the 
percentage  of  this  in  the  La  Brea  oilsand  was  as  much  as  29, 
while  in  the  Rio  Blanco  oilsand  it  was  only  0*46,  a  difference 
great  enough  to  enable  these  different  types  of  oilrock  to  be 
distinguished  easily.  Kecent  work  by  Mr.  Clifford  Richardson 
upon  the  absorptive  properties  of  fine  clays  for  bitumen 
explains  the  occurrence  of  this  percentage  of  hitherto  little- 
understood  constituent  in  asphalts,  oilrocks  and  manjaks. 
In  a  paper  read  before  the  American  Society  for  Testing 
Materials,  and  afterwards  published  in  the  "Engineering 
Record,"  he  describes  experiments  made  with  Trinidad  lake- 
asphalt  and  tests  of  the  absorptive  and  "  adsorptive  "  properties 
of  various  fine  clays  upon  solutions  of  bitumen.  The  results 
arrived  at  are  briefly  that  fine  clays  have  the  power  of 
decolorizing  bituminous  solutions  by  absorbing  or  "  adsorbing  " 
a  proportion  of  the  bitumen  in  such  a  manner  that  it  cannot 
again  be  removed  by  the  action  of  solvents.  Thus  the  greater 
part  of  the  "organic  matter  not  bitumen"  can  be  proved  to 
be  bitumen  which  cannot  be  removed  in  solution.  The 
presence  of  water  may  also  have  some  effect  in  favouring 
this  absorption,  but  the  proportion  of  fine  clay  present  seems 
to  be  the  more  important  factor.  Applying  these  results  to 
lake-pitch  and  the  oilrock  from  which  it  is  derived,  we  have  at 
once  an  explanation  of  the  presence  of  argillaceous  material 
in  the  asphalt,  and  we  must  increase  the  percentage  of  bitumen 
in  lake-pitch  by  almost,  if  not  quite,  7  per  cent,  and  the  per- 
centage in  the  oilrock  probably  by  a  much  greater  amount. 
This  makes  the  breaking  down  of  the  cohesion  of  the  oilrock 
on  exposure  much  more  intelligible. 

The  lake  itself  is,  by  the  latest  survey  made  under  the 
supervision  of  the  Inspector  of  Mines,  137  acres  in  extent,  the 
margins  being  covered  in  places  by  superficial  deposits  washed 

H 


98  OIL-FINDING 

down  from  the  surrounding  ground.  In  the  centre  the  sur- 
face of  the  asphalt  is  about  six  inches  higher  than  near  the 
sides,  and  for  some  distance  from  the  centre  there  are  no  water- 
channels.  Then  comes  a  broad  zone  characterized  by  water- 
channels  dividing  the  surface  into  roughly  circular  areas  with 
rounded  edges.  Near  the  shore  the  pitch  is  harder  as  a  rule, 
and  less  cut  up  by  water-channels.  Near  the  centre  there 
is  an  area  of  very  soft  asphalt,  where  a  little  gas  issues  slowly, 
while  there  are  similar  but  much  smaller  patches  near  the 
western  margin  and  between  it  and  the  centre.  The  dis- 
tribution of  these  areas  of  very  soft  pitch  indicates  the  proximity 
to  the  parent  oilrock,  whence  continuous  but  minute  exudation 
of  pitch  is  still  taking  place.  Lest  there  should  be  any  mis- 
understanding upon  this  point,  it  must  be  repeated  that  Messrs. 
Louis  and  Gordon  have  proved  conclusively  that  the  lake  is 
exhaustible,  and  is  being  depleted  at  a  very  rapid  rate,  but  the 
presence  of  the  patches  of  soft  asphalt,  and  the  difference  in 
level  between  the  centre  and  sides  makes  it  clear  that  additions 
of  asphalt,  probably  amounting  to  only  a  few  tons  in  the  year, 
are  still  being  made,  just  as  the  same  material  is  exuding  in  the 
ground  to  the  eastward  and  south-eastward  of  the  lake. 

The  gas  given  off  from  the  lake  is  chiefly  sulphuretted 
hydrogen  formed  by  the  action  of  water  on  sulphur  compounds 
in  the  asphalt.  It  is  seen  bubbling  up  in  the  water-channels. 
A  small  quantity  of  oil-gas,  however,  may  be  detected  issuing 
from  the  soft  patches/  V  • 

The  "  pitch-lands  "  of  La  Brea  village  are  undoubtedly,  as 
pointed  out  by  Messrs.  Louis  and  Gordon,  an  overflow  from  the 
lake.  This  overflow  has  taken  and  occupied  the  valley  of  a 
small  stream,  known  as  the  "pitch-lake  ravine,"  and  has  in 
effect  pushed  the  stream  westward,  where  it  now  flows  at  a 
higher  level  than  its  original  course.  There  is  no  evidence  of 
any  exudation  of  asphalt  in  the  village  lots,  though  gas  has 
been  detected  issuing  from  the  ground  on  one  or  two  occasions. 
Weathered  surface  deposits  underlie  as  well  as  overlie  much  of 
the  land-asphalt,  proving  that  the  overflow,  which  has  ceased 
some  years  ago,  took  place  under  subaerial  conditions. 

From  the  evidence  detailed  in  the  preceding  pages  the 
origin  of  the  Pitch  Lake  can  be  explained  as  follows : — 

In  the  first  stage  the  La  Brea  oilsand,  covered  by  its  cover- 
clay  and  succeeding  sediments,  lay  below  sea-level.  Under  a 


INDICATIONS    OF   PETROLEUM 


99 


flexuring  movement  acting  in  a  north  and  south  direction,  the 
area  was  subjected  to  elevation,  a  gentle  east  and  west  anticline 
being  gradually  formed,  and  the  strata  above  the  oilrock  were 
raised  within  the  zone  of  denudation,  though  probably  still 
below  sea  level.  Denudation  of  the  crest  of  the  anticline  took 
place  till  the  reduced  thickness  of  the  puddled  cover-clay  was 
not  sufficiently  tenacious  to  resist  the  upward  pressure  of  gas 


s. 


N. 


Sea  Level 


FIG.  4.— Stage  I. 


Sea  Level 


FIG.  5. — Stage  II.  Submarine  mud-volcano. 


Sea  Level 


FIG.  6.— Stage  III.  Formation  of  plateau. 


Sand,< 


Coverclay. 


Oil  sand  s.l 


Pitch 


FIG.  7.— Stage  IV.  Present  day. 
FIGS.  4-7. — Diagrams  to  illustrate  formation  of  Pitch  Lake,  Trinidad. 

from  the  oilrock.  A  mud  volcano  would  be  the  result,  and,  as 
denudation  and  elevation  both  continued,  would  increase  in 
size.  All  this  probably  took  place  beneath  the  water.  As  the 
covering  was  gradually  removed,  oil  began  to  exude  and  to  dry 
up  to  a  sticky  asphalt. 

About  this  time  the  anticline  was  probably  becoming  more 
clearly  denned,  and  the  site  of  the  pitch-lake  began  to  emerge 
from  beneath  the  sea  as  a  hollow  in  which  discharge  of  gas 


ioo  OIL-FINDING 

and  oil  was  continually  taking  place,  while  mingling  with 
inorganic  minerals  would  be  favoured  by  tides  and  wave  action. 
This  stage  is  marked  by  the  formation  of  the  plateau,  suggesting 
that  the"  surface  remained  at  or  near  sea  level  for  a  considerable 
time. 

As  the  land  rose  sub-aerial  denudation  would  come  into  play, 
the  oilrock  itself  being  exposed  over  a  roughly  circular  area 
defined  by  the  extent  of  the  mud  volcano.  The  anticline  being 
now  well  marked,  gas  and  oil  would  be  forced  from  all  sides 
towards  the  crest,  where  the  exposed  oilrock  would  afford  relief 
of  pressure.  The  bituminous  minerals  being  present  in  such 
quantities  in  the  oilrock  as  to  destroy  the  cohesion  of  the 
material  on  exposure,  the  solid  rock  would  gradually  crumble 
and  flow  into  the  cavity,  while  lighter  oil  and  gas  issuing  from 
below  assisted  in  the  incorporation  of  the  inspissating  petroleum 
with  the  detritus  of  the  oilrock  and  its  cover-clay  and  all  other 
material  washed  into  the  cavity.  Thus  the  basin  would  be  con- 
tinually enlarged  as  fresh  strata  of  oilrock  were  laid  open  to 
disintegration.  Convection  currents  in  the  semi-liquid  mass 
and  discharge  of  gas,  while  there  were  still  quantities  of  gas 
under  pressure  at  deeper  levels,  would  ensure  a  thorough 
mixing  of  the  different  materials  into  an  emulsion.  This  action 
is  still  going  on,  but  so  slowly  as  to  be  practically  negligible, 
while  gas  and  light  oils  have  decreased  very  greatly  in  quantity 
as  the  available  supply  of  petroleum  became  inspissated. 

Extrusion  of  semi-liquid  bitumen  proceeded  to  such  an 
extent  that  an  overflow  took  place  and  the  valley  northward 
towards  the  sea  was  completely  filled  with  asphalt,  which  is 
still  flowing  slowly  downward,  though  there  has  not  been  any 
escape  of  asphalt  from  the  lake  for  some  years.  That  this 
overflow  took  place  under  sub-aerial  conditions  is  proved  by  the 
weathered  state  of  the  superficial  deposits  beneath  the  land- 
pitch,  and  by  the  form  of  the  valley  floor.  This  shows  also 
that  the  site  of  the  lake  had  by  this  time  reached  a  considerable 
height  above  sea  level,  and  sub- aerial  denudation  must  have 
meanwhile  been  affecting  the  surrounding  country,  leaving  a 
remnant  of  the  plateau,  but  trenching  it  so  deeply  on  the  east- 
ward and  south-eastward  as  to  expose  the  oilrock,  but  not  under 
such  conditions  as  regards  gas-pressure,  etc.,  as  to  give  rise  to 
mud- volcanoes.  The  flow  and  exudation  may  at  one  time  have 
been  fairly  rapid,  but  it  is  now,  naturally,  very  sluggish,  owing 


INDICATIONS   OF   PETROLEUM  101 

to  gradual  inspissation.  In  its  latest  stage,  soil  has  actually 
been  washed  down  from  the  surrounding  country  over -the 
margins  of  the  asphalt  in  several  places." 

The  lake  is  exceeded  in  size  by  similar  asphalt  deposits  in 
Venezuela,  where,  however,  the  bituminous  material  is  purer, 
softer,  and  more  difficult  to  work  commercially.  The  depths  of 
these  Venezuelan  lakes  have  never  been  ascertained. 

Though  the  evidence  of  the  extrusion  of  asphaltic  petroleum 
as  afforded  by  pitch-lakes  is  very  striking  on  account  of  the 
concentration  of  the  material  in  one  locality,  it  is  no  more 
significant  than  the  asphaltic  deposits  that  mark  the  outcrops  of 
oil-bearing  strata  in  many  parts  of  Trinidad.  The  deposits  are 
usually  in  the  form  of  flattened  and  rounded  cones  strung  out 
in  lines  along  the  outcrop,  and  where  no  actual  exposure  of  the 
strata  is  seen  it  is  often  possible  to  map  the  outcrop  simply  by 
these  exudations.  They  vary  in  size  from  a  diameter  of  a  few 
inches  up  to  as  much  as  seven  or  eight  yards,  and  in  height 
from  an  inch  up  to  six  or  eight  feet.  Where  the  exudation  is 
rapid  and  copious  the  cones  often  coalesce,  and  an  area  of  an 
acre  or  more  may  be  completely  covered  with  the  material. 
Similarly  a  flow  of  asphalt  down  a  gully  may  be  seen  occasion- 
ally, though  it  is  seldom  that  such  streams  exceed  one  hundred 
yards  in  length  and  eight  or  ten  feet  in  depth. 

The  consistency  of  the  material  also  varies  from  soft  sticky 
oil  to  hard  compact  asphalt  that  can  be  broken  by  the  hammer, 
the  softer  varieties  being  the  most  recently  extruded.  The 
skeletons  and  remains  of  birds  and  small  animals  are  not 
unfrequently  found-  in  su<ch  soft  asphalt  deposits,  showing 
where  they  became  mired,  and  being  unable  to  extricate  them- 
selves, perished.  The  writer  on  one  occasion  found  a  live 
"  morocoi "  or  land-turtle  firmly  embedded  in  a  soft  exu- 
dation. 

There  is  nearly  always  some  evolution  of  gas  with  the 
soft  asphalt,  but  as  a  rule  it  is  discharged  very  slowly.  In 
some  of  the  cones  there  is  a  deep  crater  in  wrhich  a  bubble 
of  semi-liquid  asphalt  rises  under  pressure  of  gas  from  below 
periodically,  perhaps  once  in  two  minutes.  The  bubble  ascends 
to  the  lip  of  the  crater  where  it  breaks  by  the  formation  of 
a  small  orifice,  allowing  the  gas  to  escape  with  a  gentle  sibilant 
sound,  while  the  enclosing  film  sinks  down  again.  In  some 
parts  of  the  forests  where  the  extrusion  of  asphalt  has  been 


102  OIL-FINDING 

so  copious  that  a  thick  outcrop  of  richly  petroliferous  sand- 
stone has  been  almost  completely  sealed,  a  weird  effect  is 
produced  when  the  asphalt  cones  are  heard  sighing  around 
one ;  the  imaginative  geologist  may  weave  romances  upon 
this  slender  basis,  and  picture  the  imprisoned  oil  sighing 
for  the  advent  of  an  Exploiting  Company  by  whose  powers 
it  will  find  release. 

These  asphalt  deposits  are  always  more  or  less  mingled 
with  inorganic  and  vegetable  matter,  and  all  the  debris 
lying  on  the  surface  of  the  ground,  but  in  spite  of  this 
analysis  usually  shows  a  percentage  of  bitumen  of  70 
or  80. 

In  the  forests  of  Trinidad  the  outcrop  of  an  asphaltic 
oilsand  is  usually  marked  by  a  belt  of  stunted  vegetation, 
creepers,  and  vines  taking  the  place  of  the  larger  trees  to  a 
greater  or  less  extent. 

As  an  outcrop  becomes  clogged  by  the  exudation,  the 
sticky  oil  or  liquid  asphalt  breaks  out  again  and  again  in  the 
direction  of  dip,  so  that  we  find  the  escarpment  side  frequently 
marked  by  hard  asphalt,  while  the  fresh  exudations  are 
towards  the  dip-slope.  Such  evidence  has  more  than  once 
proved  of  value  in  giving  an  indication  of  the  direction  of 
dip  in  a  locality  where  no  exposures  of  solid  rock  were  to 
be  observed. 

It  has  often  been  suggested  that  such  asphalt  deposits 
are  not  a  favourable  sign,  that  exudation  on  a  large  scale 
must  have  depleted  the  oil-bearing  strata  beneath  the  surface, 
and  that  the  oil  will  necessarily  be  greatly  inspissated  and 
too  heavy  and  viscous.  There  is  a  modicum  of  truth  in  such 
suggestions,  but  in  actual  practice  the  geologist  need  not  fear 
such  hypothetical  depletion ;  where  he  finds  an  outcrop  steadily 
exuding  sticky  oil  and  asphalt,  he  may  be  assured  that  rich 
sources  of  oil  lie  beneath  the  surface,  and  that  inspissation, 
though  it  has  doubtless  had  a  considerable  effect  upon  the 
grade  of  the  petroleum,  will  not  have  made  it  by  any  means 
valueless.  In  California,  Mexico,  Trinidad,  and  many  other 
countries  this  has  been  proved  again  and  again.  In  fact  we 
may  consider  that  such  asphalt  deposits  are  among  the  most 
favourable  indications  that  an  oilfield  can  furnish. 

In  Trinidad  it  is  possible  to  walk  for  miles  in  the  forests 
without  ever  being  out  of  sight  of  asphalt,  and  near  Fyzabad 


Photo,  by  S.  L.  Barnes. 
1.    GROUP    OF    MUD-VOLCANOES    AT    MlNBU,    UPPER    BURMA. 


ii.  THE  LARGEST  MUD-VOLCANO  AT  MIXBU. 


Photo,  by  S.  L.  James. 


INDICATIONS    OF   PETROLEUM  103 

and  Oropuche,  and  on  Morne  L'Enfer  are  localities  where  more 
asphalt  than  soil  is  to  be  seen.  Such  deposits  are  sometimes 
worked  commercially,  but  the  difficulty  with  regard  to  them 
is  that  the  material  is  subject  to  many  local  variations,  owing 
to  greater  or  less  admixture  with  inorganic  matter,  and  to 
higher  or  lower  degrees  of  inspissation.  Consequently  the 
asphalt  requires  some  refining  to  standardize  it  before  a  com- 
mercial sample  of  constant  composition  can  be  assured.  For 
this  reason  an  asphalt  like  the  lake-pitch  of  Trinidad  with 
its  practically  invariable  composition  is  much  more  valuable 
as  a  commercial  product,  though  its  percentage  of  bitumen 
may  be  much  less  than  that  of  some  other  asphalt  deposits. 

(c)  Evolution  of  Gas. — Almost  every  seepage  of  oil  or 
exudation  of  asphalt  is  accompanied  by  a  distinct  evolution 
of  gas  in  greater  or  less  volume,  but  similar  discharges  of 
gas  may  take  place  with  little  or  no  sign  of  liquid  petroleum, 
and  it  is  to  these  that  the  term  "  gas-shows  "  is  given. 

The  most  striking  are  mud- volcanoes  (Plate  IX),  which 
must  not  be  confused  with  the  solfataric  mud-volcanoes  due 
to  true  volcanic  action.  The  mud-volcanoes  with  which  we 
are  dealing  occur  where  oil-bearing  rocks  come  near  the 
surface  but  are  covered  by  argillaceous  strata.  The  crest  of 
an  anticline  is  the  most  usual  position  as  regards  structure, 
but  favourable  conditions  for  the  formation  of  mud-volcanoes 
can  be  brought  about  in  various  ways.  Thus,  where  an  oilrock 
crops  out  amidst  a  great  thickness  of  clays,  and  the  clay  has 
been  washed  down  over  the  outcrop  thus  sealing  it  to  some 
extent,  a  mud-volcano  may  be  formed.  Again,  argillaceous 
alluvium  lying  upon  an  outcropping  oilsand  may  furnish  a 
sufficiently  impervious  cover.  Mud-volcanoes  may  also  be 
formed  along  lines  of  fault  which  permit  gas  from  underlying 
oilrocks  to  reach  the  surface,  but  this  is  a  rarer  phenomenon 
than  is  generally  supposed.  Argillaceous  outcrops  of  an  older 
series  unconformably  overlaid  by  petroliferous  strata,  may  have 
absorbed  sufficient  gas  and  petroleum  to  form  small  mud- 
volcanoes.  Lastly  the  sealing  up  of  the  outcrop  of  an  oilsand 
by  copious  extrusion  of  asphalt  may  be  so  complete  that  the 
gas  and  oil  try  to  force  their  way  through  the  outcrop  of 
cover-clay  and  form  small  mud- volcanoes.  Instances  of  the 
two  latter  cases  may  be  seen  in  Trinidad  near  Piparo  and  La 
Lune  respectively. 


104  OIL-FINDING 

It  is,  however,  on  the  crest  of  an  anticline,  where  the 
surface  is  formed  of  a  stiff  and  thick  clay,  that  the  ideal  con- 
ditions for  the  formation  of  a  large  mud- volcano  are  afforded. 
Here  the  gas-pressure  is  concentrated  continually  till  during 
dry  weather  the  surface  of  the  clay  cracks,  and  the  cracks 
gradually  extend  downwards  sufficiently  far  to  allow  a  little 
gas  to  escape.  Once  a  channel  of  exit  is  formed  it  will  pro- 
bably never  be  permitted  to  be  closed  entirely  again.  The  gas 
issuing  under  pressure  puddles  the  clay  with  the  help  of  any 
surface  water  available,  and  through  the  mud  thus  formed  the 
gas  reaches  the  surface  steadily  or  spasmodically,  carrying  a 
certain  quantity  of  the  mud  and  saline  or  oily  water  with 
it,  and  thus  in  the  course  of  time  forming  a  cone.  From 
the  evidence  afforded  by  wells  drilled  near  large  mud-volcanoes 
it  appears  probable  that  where  these  phenomena  attain  to 
any  considerable  size,  there  is  either  a  certain  quantity 
of  water  in  the  oilrock  or  there  is  a  water-bearing  band 
in  close  proximity  above  the  oilrock.  In  the  case  of  small 
cones  the  water  and  mud  are  probably  confined  to  the  zone 
nearest  the  surface;  most  clays  contain  sufficient  moisture 
to  allow  of  mud  being  formed  when  the  strata  are  disturbed 
by  discharge  of  gas,  and  surface  water  must  enter  by  the 
cracks  in  the  surface.  The  water  is  usually  slightly  saline, 
but  not  a  strong  brine. 

Professor  Carmody  has  analysed  the  water  from  the  crater 
of  a  small  mud-volcano  near  La  Lune,  Trinidad,  with  the 
following  result : — 

Total  solids   -*         ,:      .         ;         .         .     2 '506  per  cent. 

Loss  at  180  degrees  C.  (water  of  hydration 

and  ammoniacal  salts)         v        v.  |     .     0*0149 

Sodium  chloride      .         .       V        .         .     2'04 

Alkalinity  as  Na2Co3   •   -,        .  -    jjjs      .     0'38 
„  K2C03        .  .     0-40 

and  traces  of  iron,  alumina,  lime,  and  potash. 

The  water  also  contained  a  small  quantity  of  petroleum. 
This  volcano  occurs  beside  the  outcrop  of  the  Galeota  oilsand, 
where  cones  of  asphalt  cover  nearly  all  the  surface;  the 
discharge  takes  place  through  the  outcrop  of  the  cover-clay 
above  the  oil-bearing  rock.  The  cone  is  a  small  one  with  a 
crater  four  feet  in  diameter  and  full  of  water.  Two  or  three 
other  small  cones  are  to  be  observed  in  the  neighbourhood. 


INDICATIONS   OF   PETROLEUM  105 

It  is  after  a  long  drought  that  mud-volcanoes  are  generally 
most  active ;  this  is  no  doubt  due  to  the  parching  and  cracking 
of  the  clay  that  occupies  the  surface,  an  action  that  extends 
downwards  for  a  considerable  distance. 

Mud-volcanoes  are  of  all  sizes.  Of  the  number  which  the 
writer  has  observed  (nearly  one  hundred),  the  smallest  has 
a  crater  5  inches  in  diameter,  and  the  largest  a  crater  of 
150  yards  diameter.  There  is  usually  a  surrounding  belt 
of  dried  mud ;  this  has  flowed  or  been  washed  down  from  the 
crater,  which  is  often  raised  considerably  above  the  level  of 
the  surrounding  ground.  Sharp  cones  are  more  characteristic 
of  the  smaller  volcanoes,  and  are  formed  when  there  is  not  a 
super-abundance  of  water  present,  while  the  larger  volcanoes 
are  often  almost  flat  with  larger  craters  often  containing  much 
water  and  soft  mud.  The  smaller  volcanoes  are  usually  the 
most  steadily  active ;  the  larger  are  liable  to  sudden  and 
violent  eruptions  at  intervals  perhaps  of  several  years. 

All  the  phenomena  characteristic  of  true  volcanic  cones 
are  simulated ;  the  flows  of  mud  are  exactly  like  lava-streams ; 
and  when  a  cone  has  reached  a  certain  height  it  frequently 
becomes  inactive  and  another  orifice  opens  on  the  side  of  the 
cone  or  near  it.  Thus  lines  of  cones,  extinct  and  active,  are 
seen,  reproducing  on  a  small  scale  the  well-known  manifestations 
of  true  vulcanicity  along  a  fissure. 

Strewn  about  the  larger  volcanoes,  blocks  and  fragments 
of  rock,  possibly  brought  up  from  a  considerable  depth,  are 
frequently  seen.  A  little  oil  may  usually  be  detected  in  the 
water  or  liquid  mud  of  the  craters,  and  a  faint  odour  of 
petroleum  pervades  the  whole  locality,  and  is  especially 
noticeable  when  any  fragment  of  porous  rock  lying  about  the 
crater  is  broken  for  examination. 

The  best  known  and  perhaps  the  largest  mud-volcano  in 
Trinidad  is  in  Columbia  Estate  in  the  Ward  of  Cedros.  The 
usual  appearance  of  the  crater  is  a  flat  circular  area  of  dried 
mud  strewn  with  many  fragments  of  ironstone  nodules,  sand- 
stone, pyrites,  etc.  A  great  number  of  small  cones  from  a 
few  inches  up  to  two  feet  in  height  are  distributed  over  the 
expanse  of  mud,  and  these  occasionally  show  signs  of  activity. 
The  writer  once  had  the  good  fortune  to  see  this  crater  in 
eruption,  but  only  a  part  of  the  crater,  which  is  150  yards  in 
diameter,  was  explosively  active.  A  circular  orifice  of  eight 


io6  OIL-FINDING 

yards  in  diameter  filled  with  liquid  mud  had  opened  towards 
the  north-western  side  of  the  crater,. and  was  surrounded  by  a 
belt  of  half-dried  mud  some  30  yards  in  diameter,  and  raised 
above  the  surrounding  level.  At  intervals  of  about  a  minute 
the  liquid  mud  rose  in  a  huge  bubble  and  burst,  hurling  about 
a  ton  of  mud  six  or  eight  feet  into  the  air,  while  small  fragments 
torn  off  the  mass  were  thrown  some  twenty  to  thirty  feet 
upwards.  Every  minute  cone  in  the  barren  mud  area  was 
streaming  gas  and  burnt  steadily  when  set  fire  to.  The  next 
day  all  signs  of  activity  were  at  an  end. 

Sometimes  the  outbursts  of  a  mud-volcano  are  very  violent, 
especially  when  its  periods  of  activity  are  separated  by  long 
intervals  of  quiescence.  The  "  Devil's  Woodyard"  near 
Princestown  in  the  Ward  of  Savana  Grande  is  a  good  instance. 
It  received  its  name  on  account  of  the  uprooting  and  killing 
of  trees  during  an  eruption  that  took  place  in  the  first  half 
of  last  century.  When  the  writer  visited  the  locality  first, 
the  crater  was  almost  entirely  overgrown  with  vines  and  bush, 
and  a  few  small  mud-pools,  in  which  a  few  bubbles  of  gas 
could  be  detected,  were  the  only  signs  of  activity.  In  May 
1906,  there  was  a  very  violent  eruption,  which  was  said  by 
eye-witnesses  to  have  thrown  mud  over  the  tree  tops  of  the 
surrounding  forest.  After  the  outburst  the  volcano  presented 
a  very  different  appearance ;  the  crater  is  now  one  hundred 
yards  in  diameter  and  has  been  raised  five  or  six  feet,  all  traces 
of  vegetation  have  been  buried  or  blown  away,  and  blocks  of 
a  thin  band  of  fossiliferous  limestone  are  to  be  found  here 
and  there  on  the  surface  of  the  dried  mud.  A  few  very 
minute  cones  distributed  near  the  centre  of  the  crater  are 
still  active. 

Another  volcano  close  to  the  southern  coast  of  Trinidad  is 
remarkable  for  the  fact  that  a  flow  of  mud  nearly  250  yards 
in  length  stretches  from  it  to  the  beach  (Plate  X) ;  from  this 
the  name  "  Chemin  de  Diable,"  or  its  equivalent  in  the  local 
patois,  has  been  given  to  this  oilshow.  Every  10  or  12  years 
there  is  an  outburst,  which  is  evidently  very  violent  (cp. 
Plate  X),  as  blocks  of  rocks  up  to  one  foot  in  diameter  have 
been  blown  out  from  under-lying  strata,  and  trees  of  more 
than  a  foot  in  diameter  have  been  broken  off  and  the  upper 
part  hurled  away  from  the  centre  of  disturbance.  For  the 
last  few  years  this  vent  has  been  practically  quiescent,  and 


Photo,  by  S.  L.  - 

BUBBLE  BURSTING  i\  THE  CRATER  OF  THE  LARGEST  MUD-VOLCANO  AT 
MINBU,  UPPER  BURMA. 


Photo,  by.  C.  S.  Rogers, 

ii.  PART  OF  THE  CRATER  OF  A  LARGE  MUD-VOLCANO  ("CHEMIN  DE  DIABLE") 
IN  TRINIDAD,  SHOWING  TWO  MINOR  CONES. 


INDICATIONS   OF   PETROLEUM  107 

only  a  few  very  minute  cones  show  any  signs  of  activity, 
and  the  forest  is  beginning  to  encroach  upon  the  area  of 
barren  mud. 

Lagon  Bouff  in  the  Trinity  Hills  Forest  Eeserve  is  another 
well-known  and  very  active  vent.  It  lies  in  low  ground  near 
the  foot  of  the  hills,  and  consists  of  a  lake  of  liquid  mud  100 
yards  in  length  by  60  in  breadth..  It  is  in  constant  activity 
from  two  or  three  centres,  and  there  are  occasional  violent 
discharges  that  can  be  heard  some  miles  away  in  the  forest. 

Besides  these  well-known  mud-volcanoes  there  are  many 
others  of  almost  equal  importance  in  various  parts  of  the 
island.  Of  the  smaller  cones  those  of  L'Islet  Point  and  those 
at  Galfa  Point  are  perhaps  the  best  formed  and  most  typical. 

In  Burma  in  the  Districts  of  Minbu,  Thayetmyo,  Prome, 
and  Henzada,  there  are  mud- volcanoes,  most  occurring  on  the 
creste  of  anticlines,  though  some  small  ones  are  apparently 
formed  on  lines  of  fault.  Those  at  Minbu  (Plate  X)  are  the 
largest  and  best  known;  they  are  well-formed  cones  and  are 
characterized  by  steady  activity  rather  than  by  paroxysmal  out- 
bursts, owing  probably  to  the  oil-bearing  strata  lying  nearer 
to  the  surface  than  in  the  cases  of  the  large  mud-volcanoes 
described  above. 

Though  it  is  seldom  that  much  actual  oil  is  discharged 
from  mud-volcanoes,  and  it  may  not  even  be  observed  at  all 
except  where  large  pools  of  liquid  mud  and  water  fill  the 
craters,  there  is  no  doubt  as  to  the  presence  of  oil  beneath  the 
surface.  The  only  instances  that  the  author  has  seen  of  oil- 
wells  drilled  near  such  gas-vents  have  all  been  successful  in 
striking  oil. 

Evolution  of  gas  often  occurs  without  the  formation  of  a 
mud- volcano,  especially  where  the  strata  are  hard  or  sandy, 
but  it  may  also  take  place  from  a  clay  outcrop.  One  interesting 
example  of  this  may  be  seen  in  the  Ward  of  Oropuche,  Trinidad, 
where  gas  issues  steadily  from  the  clay  soil  over  an  area  of 
about  a  square  yard.  This  show  is  situated  about  the  crest 
of  the  Central  (Western)  Anticline.  The  land  has  been  cleared 
for  cultivation  recently,  and  something  in  the  nature  of  a  regular 
vent  is  forming.  In  the  course  of  time  it  will  probably  become 
a  small  mud- volcano. 

Gas-wells,  small  pools  of  water  disturbed  by  steady 
evolution  of  gas,  are  not  unfrequent  occurrences  in  oilfields, 


io8  OIL-FINDING 

and  the  volume  of  gas  is  sometimes  sufficient  to  be  used 
continuously  as  a  source  of  light  and  heat.  The  "Boiling 
Spring "  in  Barbados  (Scotland  District),  is  well-known ;  it 
is  kept  in  a  constant  ebullition  by  the  gas  from  an  oilsand 
bubbling  through  the  water.  Near  Guayaguayare,  in  Trinidad, 
there  are  similar  bubbling  springs,  the  gas  from  which  burns 
steadily  when  ignited. 

All  these  gas- shows,  whether  in  the  form  of  a  great  mud- 
volcano  or  little  gas-pools,  are  very  important  evidence,  as 
without  sufficient  gas-pressure  an  oilfield  may  be  very  expensive 
to  work  and  the  wells  may  not  have  a  long  life. 

Another  form  of  gas-show  which  is  sometimes  a  very  helpful 
sign  is  the  evolution  of  sulphuretted  hydrogen.  This  may  not 
be  connected  with  petroleum  at  all,  but  in  many  oilfields  this 
gas,  only  too  readily  detected  by  its  odour  and  its  action  upon 
metallic  silver,  is  formed  by  the  action  of  water  upon  sulphur 
compounds  in  the  petroleum  and  its  inspissated  residues. 
Where  the  oil-bearing  rock  is  a  limestone,  as  in  some  of  the 
"  sour "  oilfields  of  Ohio  and  Indiana,  discharge  of  hydrogen 
sulphide  is  not  uncommon  from  the  oil-bearing  series.  The 
evolution  of  this  gas  may  be  so  copious  as  to  be  dangerous  to 
life.  At  Marmatain  in  Persia,  where  a  sulphurous  oil  in  the 
limestone  bands  forms  this  gas  under  weathering  processes,  two 
Persians  lost  their  lives  by  going  to  bathe  in  a  pool  in  a  small 
gully  where  the  gas  had  collected  on  a  still  day  to  such  an 
extent  that  it  overcame  them  ;  the  bodies  were  not  discovered 
till  next  day.  In  the  prolific  field  of  Maidan-i-Naphtun  also, 
one  of  the  wells  gave  a  gas  with  a  large  proportion  of 
sulphuretted  hydrogen,  and  birds  and  jackals  were  found  after  a 
still  night  dead  near  the  derrick. 

(d)  Outcrops  of  Bituminous  Strata. — Even  when  no  seepage 
of  oil,  exudation  of  asphalt,  or  evolution  of  gas  is  to  be  observed, 
it  is  generally  possible  to  recognize  an  oilrock  by  its  outcrop. 
With  an  oil  of  asphaltic  base  this  is  a  simpler  matter  than 
when  paraffin  oils  are  dealt  with.  In  the  former  case  there 
is  usually  at  least  a  slight  bituminous  impregnation  or  dis- 
coloration, and  the  odour  of  petroleum  may  be  detected  even 
when  there  is  very  little  coloration.  Oilsands  are  often  so 
highly  impregnated  that  even  when  the  oil  is  dried  up  by 
inspissation  at  the  surface  the  bituminous  content  is  so  high 
that  the  rock  can  hardly  be  broken  by  the  hammer,  but  can  be 


INDICATIONS    OF   PETROLEUM  109 

dented  or  cut,  and  small  projections  can  be  twisted  off  in  the 
fingers  and  rolled  into  pellets  in  the  hand.  There  are  large 
areas  in  Trinidad  covered  by  outcrops  of  this  kind,  and  the 
material  has  been  quarried  for  use  on  roads ;  the  rock  crushed 
under  traffic  forms  a  smooth  surface  that  does  not  wash  away 
easily  during  rains,  nor  become  hard  and  slippery  in  cold  and 
wet  weather  as  does  an  asphalt  surface.  The  "  tar-sands  "  of 
Barbados  are  precisely  similar,  though  not  always  so  highly 
impregnated. 

But  when  an  outcrop  has  been  subjected  to  weathering  for 
long  periods  without  fresh  access  of  oil  or  bitumen,  it  may  show 
very  little  trace  of  a  former  impregnation.  In  such  cases  the 
mode  of  weathering  or  the  traces  of  sulphur  compounds  may  be 
sufficient  to  prove  that  we  are  dealing  with  an  oilrock.  Any 
sand  may  be  an  oilrock,  but  if  in  examining  a  section  one  finds 
certain  bands  softer  and  less  coherent,  darker  in  colour,  and 
with  rounded  contours  as  compared  with  otherwise  similar 
sandstones  in  the  same  section,  it  may  he  presumed  that  if  any 
of  the  strata  have  been,  or  are  beneath  the  surface,  oil-bearing, 
it  is  these,  and  if  followed  up  in  the  field  and  studied  under 
different  conditions  as  regards  structure  and  exposure,  clear  and 
unmistakable  evidence  may  be  forthcoming. 

Faint  yellow  stains  or  flecks  due  to  traces  of  sulphur  from 
decomposed  sulphur  compounds  often  afford  additional  evidence, 
and  may  be  the  last  remaining  traces  of  a  former  impreg- 
nation. 

The  coloration  due  to  metallic  oxides  or  sulphides,  iron 
or  manganese  compounds,  may  in  some  cases  simulate  a 
coloration  due  to  bitumen,  but  when  the  rock  is  crushed  and 
washed  or  vanned,  or  treated  with  a  solvent  such  as  benzine, 
there  can  be  no  mistake  as  to  the  nature  of  the  colouring 
material. 

With  oils  of  a  paraffin  base  there  may  be  no  such  evidence-, 
and  when  a  weathered  outcrop  is  suspected  of  having  been  im- 
pregnated, it  is  necessary  to  break  open  any  nodules  or  hard 
and  compact  bed  that  the  strata  may  contain  to  search  for 
traces  of  petroleum.  The  more  compact  and  fine-grained  the 
material,  the  less  easily  will  any  impregnation  be  removed  by 
weathering,  so  a  survival  of  an  impregnation  may  be  discovered 
in  a  hard  nodular  band,  when  the  surrounding  more  porous  and 
once  more  highly-impregnated  strata  have  lost  all  trace  of  the 


no  OIL-FINDING 

former  presence  of  oil.  A  faint  odour  of  vaseline  is  often  the 
only  evidence  that  can  be  obtained.  In  Trinidad  the  oils  of 
paraffin  base  occurring  in  thin  sands  among  thick  masses  of 
stiff  clay  frequently  betray  their  presence  by  the  residues  of  an 
impregnation  and  the  unmistakable  odour  of  vaseline  in 
nodules  of  iron  and  lime  carbonates  found  in  the  clay.  In 
Burma  also,  where  paraffin  oils  are  the  rule,  the  oilrocks  at 
outcrop  frequently  show  no  trace  of  petroleum,  and  compact  or 
nodular  bands  have  to  be  examined. 

Where  the  oilrock  is  a  limestone  there  may  be  no  signs  of 
petroleum  at  outcrop,  but,  as  pointed  out  already,  crystals  of 
sulphur  or  evolution  of  sulphuretted  hydrogen  may  be  sufficient 
to  point  to  the  former  presence  of  an  oil  containing  sulphur 
compounds.  The  staining  of  pebbles  in  a  stream  by  the  deposi- 
tion of  sulphides,  and  the  presence  of  finely  divided  sulphur  in 
the  water,  giving  it  a  milky  appearance,  are  pieces  of  evidence 
that  very  frequently  characterize  outcrops  of  limestones  or 
shale  that  contain  or  have  contained  an  oil  with  a  percentage  of 
sulphur. 

(e)  Manjak  and  Ozokerite  Veins. — No  account  of  indications 
of  petroleum  would  be  complete  without  some  mention  of  the 
veins  of  solid  petroleum  residues,  known  by  various  names  in 
different  countries,  and  according  to  differences  in  their  com- 
position. The  solid  bitumens,  though  all  closely  allied  in  com- 
position, differ  greatly  in  physical  characters  such  as  lustre  and 
jointing,  while  in  such  practical  matters  as  melting  point, 
purity  and  efficiency  as  insulating  material  in  electrical  work, 
there  are  also  many  differences.  In  the  United  States  Gilsonite 
and  Uintaite  are  the  prevalent  names,  while  a  hard  and  much 
altered  form  is  known  as  Grahamite.  In  Canada  Albertite  is 
the  designation  of  a  very  hard  variety.  But  every  gradation 
between  the  hardest  and  most  mineralized  form  and  a  viscous 
pure  bitumen  can  be  discovered.  The  author  prefers  to  use  the 
old  name  Manjak  or  Munjac  as  a  generic  term  for  all  these 
bituminous  minerals ;  the  term  has  been  in  use  in  Barbados 
since  early  in  the  seventeenth  century. 

Ozokerite  is  to  an  oil  of  paraffin  base  what  manjak  is  to 
an  oil  of  asphaltic  base,  but  though  there  are  many  varieties 
of  ozokerite  the  mineral  is  less  common  than  the  solid -bitu- 
minous minerals,  and  the  term  is  applied  to  all  grades 
of  mineral  wax.  The  origin  of  these  minerals  is  the  same; 


INDICATIONS   OF   PETROLEUM  in 

they  are  the  solid  residues  from  the  inspissation  of  petroleum 
beneath  the  surface,  and  may  be  looked  upon  as  intrusive 
petroleum. 

Though  it  may  not  be  possible  in  all  cases  to  prove  that 
rnanjak  veins  are  essentially  phenomena  of  an  oilfield  or  the 
margin  of  an  oilfield,  their  association  with  petroleum  has  been 
established  so  frequently,  and  they  afford  in  many  instances 
such  valuable  indications  as  to  where  the  search  for  oil  is  likely 
to  be  successful,  that  we  must  regard  the  study  of  the  manjak 
group  of  minerals  as  part  of  the  necessary  knowledge  with 
which  the  geologist  who  has  to  specialize  in  oilfield  work  must 
make  himself  familiar. 

The  important  points  to  be  noted  are  the  conditions  under 
which  veins  of  manjak  are  found.  Briefly  put,  manjak  veins 
occur  where  a  thick  series  of  strata,  partly  or  wholly  of 
impervious  material,  overlies  a  source  of  asphaltic  oil,  and 
where,  either  due  to  the  softness  of  the  superincumbent  rock, 
to  contraction  owing  to  partial  drying,  or  to  earth-movement, 
planes  of  weakness  have  been  developed  enabling  intrusion  of 
petroleum  from  below  to  take  place.  Manjak  veins  are 
invariably  highly  inclined  or  even  vertical,  except  where  small 
local  off-shoots  from  a  larger  vein  may  take  gentler  inclinations. 
Bedding-planes,  fault-planes,  joints  or  minor  slip-planes  in  an 
argillaceous  mass  afford  opportunities  for  this  intrusive  action. 
The  occurrence  along  bedding  planes  has  more  than  once  led 
to  the  belief  that  manjak  is  of  the  nature  of  coal,  and  a  mode 
of  formation  by  the  sinking  of  heavy  tropical  timber  to  form 
a  deposit  in  water  of  not  more  than  one  hundred  fathoms  in 
depth  has  actually  been  suggested  and  published.  Quite  apart 
from  its  inherent  improbability,  such  a  theory  fails  at  once  when 
the  facts  are  studied  in  the  field.  The  occurrence  of  manjak 
among  foraminiferal  clays,  e.g.  in  the  San  Fernando  Manjak  field, 
is  hardly  compatible  with  a  drift-origin  theory,  while  the  fact 
that  the  veins  cross  the  bedding  in  all  directions,  and  only 
occasionally  run  along  it  for  short  distances,  proves  that  the 
mineral  is  not  a  deposit  and  must  have  reached  its  present 
position  in  some  other  manner.  In  the  United  States  Mr. 
Eldridge  has  described  vertical  veins  of  gilsonite  which  have 
been  traced  for  great  distances  through  horizontal  or  nearly 
horizontal  strata,  which  are  trenched  by  great  canons;  the 
orientation  of  these  veins  varies  very  little,  and  may  be  due 


112  OIL-FINDING 

either  to  earth-movement  or  to  the  drying  of  the  strata  caused 
by  proximity  to  the  canons. 

In  Trinidad  and  Barbados  the  mineral  occurs  in  thick 
masses  of  argillaceous  strata  and  slip-planes,  joint-planes,  and 
occasionally  bedding-planes  determine  the  directions  of  the 
veins,  but  irregular  pockets  are  developed  here  and  there.  The 
veins  vary  in  thickness,  orientation,  and  dip,  but,  as  stated 
before,  are  nearly  always  highly  inclined.  Perhaps  the  largest 
vein  of  manjak  that  has  ever  been  described  is  the  Vistabella 
Vein  in  the  San  Fernando  Manjak  field.  It  attains  a  thickness 
of  thirty-three  feet  for  part  of  its  course. 

Manjak  varies  considerably  in  purity  and  composition, 
according  to  the  environment  in  which  it  is  found.  It  is  usual 
in  testing  a  manjak  to  treat  it  with  petroleum  ether,  which 
removes  in  solution  a  percentage  which  is  called  "  petroletie," 
while  the  insoluble  percentage  is  called  "  asphaltene."  The 
most  valuable  types  are  jet  black,  bright  and  lustrous,  with  a 
beautiful  conchoidal  fracture  and  a  high  percentage  of  petrolene. 
Small  percentages  of  water,  volatile  matter,  and  inorganic 
impurities,  are  always  present.  The  .quality  of  a  sample  is 
determined  by  its  freedom  from  impurities  and  its  percentage 
of  petrolene,  since  a  high  proportion  of  the  latter  enables  the 
solid  bitumen  to  be  fluxed  more  readily. 

Columnar  jointing  is  a  frequent  phenomenon  in  veins  of 
manjak,  and  it  may  extend  across  the  whole  vein,  the  columns 
being  at  right  angles  to  the  sides.  The  columnar  variety  is 
usually  poorer  in  petrolene  than  the  variety  with  conchoidal 
fracture ;  it  has  also  a  duller  lustre  and  a  coaly  fracture.  The 
structure  is  due  to  the  loss  of  volatile  constituents.  Every 
phenomenon  of  an  intrusive  dyke  or  vein  of  igneous  rock 
is  simulated  by  these  intrusive  bitumens,  and  veins  may  be 
seen  with  margins  of  columnar  structure  and  a  central  portion 
of  the  lustrous  conchoidal  variety,  which  represents  a  later 
intrusion.  The  percentage  of  petrolene  also  increases  towards 
the  centre  of  every  vein,  and  further  increases  in  analogous 
parts  of  the  same  vein  as  it  is  traced  to  deeper  levels,  while 
a  vein  that  does  not  crop  out  at  the  surface  generally 
contains  a  greater  proportion  of  petrolene  than  one  that  is 
exposed.  These  facts  prove  that  there  is  a  gradual  loss  of 
volatile  constituents,  and  a  gradual  drying-up  or  inspissation 
of  the  mineral  towards  the  sides  of  the  vein  and  towards  the 


INDICATIONS    OF   PETROLEUM  113 

surface.  Thus  a  specimen  from  the  50-foot  level  in  Marbella 
Mine  can  be  compared  with  a  specimen  from  the  same  vein 
at  a  depth  of  125  feet,  the  analyses  being  by  Professor 
Carmody : — 

From  50  From  125 

feet  level  feet  level 

Water         .         .       ':"  >  0'65 .  .  TO 

Organic  matter  .         »  .  94'SO  .  96'20 

Mineral      tj      .         .  .  4-55.  .  2'80 

Percentage  of  petrolene  .  8*80.  ,  9'6 

Specimens  from  deeper  levels  in  the  Vistabella  Mine  gave 
percentages  of  petrolene  up  to  15 '2. 

In  Barbados,  where  many  of  the  veins  do  not  crop  out  at 
the  surface,  even  higher  percentages  of  petrolene  are  recorded. 
One  vein  gave  18  per  cent,  from  its  columnar  selvage  and  35 
per  cent,  from  the  central  portion. 

The  clays  surrounding  manjak  veins  are  often  seen  to 
contain  sticky  inspissated  oil  or  liquid  asphalt  along  joint 
faces  and  slip-planes,  and  nodules  of  clay-ironstone  slightly 
more  porous  than  the  clay  show  abundant  evidence  of  impreg- 
nation. From  the  centre  of  a  vein  with  columnar  jointing  in 
Marbella  Mine  the  writer  has  seen  a  semi- solid  bitumen  slowly 
extruding.  This  material  was  brittle  enough  to  be  broken  up 
by  a  sharp  tap,  but  could  be  bent  and  twisted  without  breaking 
if  pressure  was  applied  slowly.  Its  percentage  of  petrolene 
was  56  ;  it  is  a  later  intrusion. 

From  this  evidence  it  is  obvious  that  the  mineral  has  been 
introduced  in  a  liquid  or  semi- liquid  state,  and  has  gradually 
dried  and  hardened  in  situ.  A  still  more  convincing  piece  of 
evidence  is  the  fact  that  sometimes  when  a  vein  is  followed  to 
a  considerable  depth  it  is  found  to  end  in  a  sand  or  sandstone 
fully  impregnated  with  sticky  oil,  "tar-sand"  as  it  is  called 
in  Barbados.  This  makes  the  origin  of  the  mineral  quite  clear, 
and  its  relations  to  petroleum  on  a  larger  scale  can  usually  be 
established  by  field  evidence.  Thus  in  the  San  Fernando  Manjak 
field  an  oilsand,  with  several  "  shows  "  of  heavy  oil  on  its  out- 
crop dips  steeply  beneath  the  clay  beds  in  which  the  manjak 
is  worked  (Fig.  8),  and  presumably  underlies  these  strata 
throughout  the  syncline.  The  shaded  part  in  the  diagrammatic 
section  shows  the  zone  in  which  manjak  veins  have  been  proved 
by  mining.  It  is  natural  to  expect  that  the  crest  of  an  anticline 

I 


114 


OIL-FINDING 


would  be  the  most  likely  place  to  find  veins  of  manjak,  and 
small  veins  have  certainly  been  discovered  on  or  near  anticlinal 
crests  where  a  considerable  thickness  of  impervious  argillaceous 
strata  lies  above  the  oilrocks,  e.g.  in  the  Poole  District  and  near 
the  "Devil's  Woodyard,"  in  Trinidad,  but  the  centre  of  a 
sigmoidal  flexure,  between  syncline  and  anticline  seems  to  have 


WATER  LEVEL 


FIG.  8. — Diagram  illustrating  mode  of  occurrence  of  Manjak  veins  in  Trinidad 
(San  Fernando  Field).  1.  Cretaceous  inlier;  2.  Oil-bearing  sand; 
3.  Clay;  4.  Sandstone;  5.  Zone  containing  Manjak  veins. 

some  special  advantages  that  make  it  eminently  favourable 
to  the  intrusion  of  these  bituminous  minerals.  Probably  the 
strains  developed  during  the  earth-movement  that  caused  the 
flexures  have  resulted  in  slip-planes  in  the  argillaceous  strata 
and  so  favoured  the  intrusive  action.  The  pressure  of  gas 
occluded  in  or  associated  with  the  oil  was  probably  the  moving 
force. 

On  a  minute  scale  intrusion  from  the  upper  surface  of  an 
oil  rock  may  frequently  be  seen.  Where  the  La  Brea  Oil-bearing 
Group  is  exposed  in  coast-section  near  the  Pitch  Lake,  small 
veins  of  asphalt  may  be  observed  extending  vertically  upwards 
from  the  upper  surface  of  the  petroliferous  sand,  and  hand 
specimens  may  even  be  obtained  of  such  veins  not  more  than 
an  inch  in  thickness  with  portions  of  the  country  rock  on  each 
side.  This  is  sufficient  to  suggest  the  possibility  of  similar 
intrusions  on  a  much  larger  scale,  given  the  requisite  conditions. 

Ozokerite  veins  occur  under  much  the  same  conditions,  but 


INDICATIONS   OF   PETROLEUM  115 

seldom  attain  to  the  same  size  and"  thickness  that  manjak  veins 
reach. 

Paraffin  oils  being  as  a  rule  more  mobile  and  lighter,  and 
containing  less  material  capable  of  forming  solid  residues  than 
asphaltic  oils,  are  liable  to  find  their  way  further  without 
solidifying  as  they  gradually  become  inspissated,  and  are  not 
likely  to  coagulate  in  such  large  masses.  Consequently  thin 
veins  and  networks  of  veins,  and  bands  of  porous  inorganic 
material  impregnated  with  the  solid  paraffin  wax  are  more 
frequent  than  thick  well-defined  intrusions.  The  colour  of  the 
ozokerite  varies  from  yellowish  white  to  brown  and  black,  but 
the  latter  colours  are  by  far  the  most  common.  The  mineral, 
being  of  considerable  value,  is  frequently  mined,  but  the  mines 
do  not  often  become  great  commercial  successes  owing  to  the 
lack  of  thick  and  solid  veins. 

The  occurrence  of  rock-salt  or  brine-springs  is  not  dealt 
with  as  evidence  of  petroleum,  although  the  association  of  brine 
or  salt  with  oil  is  frequent  in  many  parts  of  the  world.  In  a 
former  chapter  it  was  shown  that  this  may  not  be  an  essential 
association,  but  another  indirect  effect  of  the  same  cause ;  con- 
sequently though  the  search  for  brine  has  often  led  to  the  find- 
ing of  oil,  and  its  occurrence  may  often  give  valuable  evidence 
to  the  geologist,  it  is  hardly  justifiable  to  class  rock-salt  or 
brine  with  surface  indications  of  petroleum. 

All  the  phenomena  described  above  must  be  noted  by  the 
geologist,  and  the  significance  of  each  learnt,  so  that  he  may 
be  able  to  ascertain  whether  or  no  a,  series  is  or  has  been 
petroliferous;  evidence  may  be  very  scanty  in  jungle- covered 
ground,  and  he  may  have  to  rely  upon  very  meagre  indications, 
which  might  easily  be  overlooked.  It  is,  therefore,  necessary 
that  every  variety  of  indication  should  be  familiar  to  him.  In 
argillaceous  strata  he  must  be  especially  on  the  alert ;  where 
exposures  are  few  and  dips  unreliable,  a  minute  gas-show,  or 
the  discovery  of  a  few  fragments  of  manjak,  may  be  of  great 
value  in  assisting  him  to  determine  where  a  test  well  should  be 
located. 

2.  Indications  in  a  Borehole. — Evidence  that  may  be  con- 
sidered as  "favourable,"  and  as  pointing  to  the  prospect  of 
striking  oil  in  a  drilled  well,  may  be  of  almost  any  nature,  and 
such  evidence  can  only  be  interpreted  by  reference  to  what  is 
known  of  the  geological  formation  or  series  that  is  being  tested. 


n6  OIL-FINDING 

During  the  first  tests  of  a  new  presumed  oilfield,  where  perhaps 
little  or  nothing  is  known  of  the  geology  of  the  district,  a  state 
of  things  which  even  nowadays  may  be  met  with  only  too 
often,  "  shows  "  of  gas  and  oil  are  really  the  only  favourable 
indications  that  can  be  recorded.  And  even  these  may  be 
entirely  deceptive,  for  it  may  be  that  such  shows  are  derived 
from  horizons  which  in  other  districts  are  represented  by  thick 
and  prolific  oilrocks,  but  which  have  thinned  out  to  insignifi- 
cant streaks  in  the  area  being  tested.  And  the  driller  may  be 
tempted  to  drill  deeper  and  deeper  into  strata  that  are  not  and 
never  have  been  petroliferous.  The  well  may  even  pass  through 
an  unconformability  into  some  lower  series,  which,  if  exposed 
at  the  surface,  would  never  be  tested  for  petroleum  even  by 
that  most  hopeful  of  optimists,  the  driller  of  wild-cat  wells. 
Yet,  because  light  shows  of  oil  and  gas  were  encountered  at 
some  stage  or  stages,  the  well  may  be  continued  for  months  at 
ruinous  expense. 

On  the  other  hand,  when  the  geology  of  a  district  has  been 
carefully  worked  out,  when  the  strata  to  be  drilled  through 
are  known,  and  the  depth  to  be  drilled  estimated  approxi- 
mately, a  "  favourable  indication  "  consists  of  any  evidence  that 
shows  that  the  strata  to  be  tested  are  being  approached,  and 
the  fact  that  no  shows  of  oil  or  gas  are  encountered  may  be  a 
favourable  indication,  proving  that  the  petroliferous  contents  of 
the  strata  beneath  are  securely  sealed  beneath  an  impervious 
cap  and  that  migration  upwards  has  been  prevented. 

The  recognition  of  any  known  baud  of  rock  in  the  log  of 
the  well,  or  by  fragments  from  the  bailer,  even  if  it  be  a  prolific 
water-sand,  which  will  enable  the  depth  to  the  oil-bearing 
horizon  to  be  re-estimated,  is  often  of  great  importance,  as 
where  lateral  variation  in  rock  groups  is  the  rule  estimates  of 
thickness  made  from  some  section  at  a  distance  can  never  be 
very  accurate. 

When  one  or  two  wells  have  been  drilled,  information  from 
the  boring  journals  should  be  sufficient  to  enable  the  geologist 
to  judge  whether  the  prospects  of  a  third  well  are  promising  or 
not,  and  the  depth  can  be  calculated  with  a  fair  degree  of 
accuracy  if  the  area  has  been  geologically  mapped.  But  with- 
out a  large  scale  geological  map  the  boring  journals  are  of  very 
little  use  unless  the  wells  are  close  together.  In  the  author's 
experience  estimates  of  the  depths  to  be  drilled  have  come 


INDICATIONS   OF   PETROLEUM  117 

within  10  feet  of  the  actual  depth  in  the  case  of  new  wells  two 
miles  distant  from  any  previous  well,  and  for  depths  of  nearly 
2000  feet,  in  an  area  of  great  and  sharp  flexures.  Such 
estimates  were  arrived  at  by  careful  six-inch  mapping  and 
making  allowance  for  the  thinning  of  rock  groups  owing  to  an 
ascertained  lateral  variation. 

Oil  is  seldom  struck  without  any  warning ;  light  gas 
"  shows  "  or  light  shows  of  filtered  oil  and  gas  often  occur  at 
some  distance  above  the  actual  oilrock.  These  are  due  to  a 
gradual  migration  from  below.  Gas  is  not  necessarily  a  hopeful 
indication,  but  „  when  gas-pressure  increases  steadily  as  the 
drill  penetrates  deeper  and  deeper  into  a  fairly  impervious 
group  of  strata,  it  may  be  taken  as  a  very  favourable  sign ;  the 
first  porous  band  of  any  thickness  met  with  will  probably  be 
oil-bearing.  Even  in  such  a  case,  however,  the  oilpool  may  be 
missed  and  the  oilsand  found  to  be  full  of  water.  An  example 
of  this  occurred  in  Trinidad.  A  light  show  of  oil  was  struck  at 
shallow  depth  and  cased  off;  the  well  was  continued  and  struck 
strong  gas  in  a  sandy  shale.  The  gas-pressure  continued  to 
increase  as  the  boring  proceeded,  and  caused  much  difficulty  in 
the  drilling.  At  greater  depths,  however,  the  gas-pressure 
began  to  decrease,  and  when  the  well  reached  a  thick  sand-bed  \ 
it  was  found  to  contain  salt  water.  The  gas  had  reached  the  ] 
locality  by  lateral  migration. 

It  is,  of  course,  when  the  first  tests  of  a  new  field  are  being 
drilled  that  indications  become  most  important,  and  especially 
when  unknown  strata  are  being  penetrated.  Though  a  district 
may  be  mapped  geologically  with  great  care,  and  the  series 
proved  to  be  petroliferous,  the  first  well  may  be  drilled  into 
strata  that  are  not  exposed  for  a  distance  of  many  miles  from 
the  locality.  A  study  of  the  lateral  variation  may  have  made 
it  appear  highly  probable  that  oil-bearing  strata  are  beneath 
the  surface,  the  geological  structure  may  be  eminently  favour- 
able, and  the  well  carefully  located,  but,  as  the  depth  to  be 
drilled  is  unknown  or  only  roughly  estimated,  there  is  neces- 
sarily some  uncertainty.  It  is  in  such  circumstances  that  the 
evidence  from  the  log  must  be  most  carefully  studied.  Any 
light  show  of  gas  or  oil,  if  in  thin  beds,  will  be  a  favourable 
sign.  But  if  thick  porous  beds  are  pierced  with  light  shows  of 
gas  or  oil  accompanied  by  water,  the  indication  is  most  un- 
favourable. If  the  drill  has  passed  through  a  great  thickness 


u8  OIL-FINDING 

of  stiff  argillaceous  strata,  when  it  first  reaches  a  porous  bed 
important  evidence  will  be  forthcoming ;  if  oil  appears  in  the 
bed  the  indication  is  most  hopeful,  but  if  water,  the  prospects 
of  the  well  are  gloomy.  The  nature  of  the  argillaceous  strata 
have  also  to  be  considered  ;  if  they  are  typically  marine  through- 
out, the  prospects  will  not  be  quite  so  good  as  if  estuarine  con- 
ditions are  indicated  by  the  presence  of  gypsum  or  selenite  at 
some  horizons,  and  especially  towards  the  base  of  the  argil- 
laceous group. 

Alternating  bands  of  clays  and  sandstones  may  be  regarded 
as  moderately  favourable,  even  if  the  sands  contain  water. 
Nodules  of  clay-ironstone,calcareous  concretions  in  sandstones, 
glauconitic  sands,  and  all  the  characteristics  of  estuarine  and 
deltaic  beds  may  be  regarded  as  favourable. 

Beds  of  coal  or  lignite,  if  pierced  at  comparatively  shallow 
depths  where  comparatively  thick  clays  underlie  them,  are 
hopeful  indications  if  the  geological  structure  be  good;  if 
struck  at  great  depths,  the  field  will  probably  have  to  be 
abandoned. 

Beds  of  gypsum  or  rock-salt  are  indifferent  evidence;  oil- 
bearing  strata  are  not  infrequently  found  below  them,  but  just 
as  frequently  above  them,  while  in  many  cases  they  are  not 
associated  in  the  same  series  with  petroleum. 

The  occurrence  of  marine  limestone  is,  generally  speaking,  a 
bad  sign,  though  many  prolific  fields  have  a  limestone  as  their 
reservoir  rock.  An  entirely  marine  series,  without  intercala- 
tions with  littoral  or  estuarine  beds,  is  to  be  avoided. 

Fresh  arkoses  or  grits  containing  fresh  felspars,  micas  or 
volcanic  material,  are  usually  unfavourable  as  indicating  the 
proximity  of  crystalline  rocks  or  volcanic  strata  which  were 
being  denuded  while  the  series  was  being  deposited.  The 
approach  to  an  unconformability,  however,  which  may  be 
indicated  by  the  presence  of  conglomerates  formed  of  pebbles 
derived  from  an  older  series  is  often  worth  noting,  as  the  basal 
arenaceous  groups  of  a  series  are  frequently  oil-bearing  under 
favourable  conditions.  The  reason  of  this  is  obvious  when  we 
consider  the  landward  margins  of  a  delta,  and  the  probability 
of  the  formation  of  swamps  between  the  main  mouths  of  a 
river  and  the  higher  ground  that  may  bound  the  delta  on  one 
or  both  sides.  Pebbles  or  fragments  of  pebbles  may  frequently 
be  brought  up  in  the  bailer,  and  a  bed  consisting  chiefly  of 


INDICATIONS    OF   PETROLEUM 


119 


pebbles  can  be  recognized  by  any  competent  driller,  so  there 
should  be  no  difficulty  in  ascertaining  the  presence  of  con- 
glomerates. 

If  a  thick  arenaceous  series,  whether  conglomeratic  or  not, 
is  being  drilled,  and  salt  water  is  found  in  it,  there  is  little 
hope  of  an  oilwell  till  some  underlying  impervious  rock  group 
is  reached  and  drilled  through. 


FAVOURABLE. 


UNFAVOURABLE. 


Always. 

Usually. 

Sometimes. 

Usually. 

Always. 

Shows     of     oil 

Shows   of    fil- 

Shows   of    oil 

Light  shows  of 

with      strong 

tered  oil  with 

with        very 

oil    in    thick 

gas    in     thin 

'gas. 

little  gas. 

porous     beds 

porous     beds 

with  water  or 

among  imper- 

brine. 

vious  strata. 

Evidence      of 

Evidence  of  en- 

estuarine   or 

tirely  marine 

deltaic    con- 

conditions. 

ditions. 

Beds   of    gyp- 

sum or  rock- 

salt. 

Brine. 

Shows   of  gas 

< 

below  or  in  a 

thick  argilla- 

ceous series. 

Shows  of  par- 

Shows of  par- 

tially inspis- 
sated oil  near 

-    - 

tially  inspis- 
sated oil  deep 

the  surface. 

down. 

Water  -  sands 

below  a  thick 

argillaceous 

series. 

Lignites        or 
coals,     fossil 

Sulphuretted 
hydrogen  ac- 

Hot water  with 
neither  oil  nor 

resin,        sul- 

companied by 

gas. 

phur  or  sul- 

hot  water. 

phure  tted 

, 

hydrogen. 

Gas  in  slightly 

Gas-shows  ac- 

porous  strata, 

companied 

with  pressure 

by    water   in 

increasing 

porous     beds 

downwards. 

among      im- 

pervious beds. 

Ozokerite     or 

manjak  veins. 

120  OIL  FINDING 

But  when  all  is  said  and  done,  every  case  must  be  con- 
sidered on  its  merits  by  reference  to  what  is  known  of  (1)  the 
geology  of  the  district  or  country  ;  (2)  the  stratigraphy  of  the 
series  that  is  being  tested,  and  (3)  the  geological  structure  in 
the  particular  locality.  An  indication  may  be  exceedingly 
favourable  where  the  structure  is  not  very  attractive,  while  in 
a  field  with  ideal  geological  structure  it  might  give  by  no 
means  a  hopeful  prediction  as  to  the  results  likely  to  be 
obtained. 

It  is  therefore  almost  impossible  to  tabulate  what  are,  or 
are  not,  hopeful  indications,  and  the  table  on  p.  119  must  be 
regarded  only  as  a  rough  guide  to  the  geologist  who  has  to 
study  well  records  in  a  new  field.  It  is  presumed  that  the  well 
has  been  located  where  the  geological  structure  is  favourable. 


CHAPTER   VII 
STRATIGRAPHY 

IN  the  foregoing  chapters  an  account  has  been  given  of  the 
principal  subjects  which  the  prospecting  geologist  must  study 
in  the  field  before  he  will  be  thoroughly  competent  to  advise 
a  company  in  the  exploitation  of  petroleum.  The  necessity  of 
elucidating  lateral  variation  has  been  dealt  with,  the  working 
out  of  geological  structure  has  been  treated  at  some  length,  and 
the  various  kinds  of  evidence  upon  which  a  series  can  be 
determined  to  be  petroliferous  have  been  described.  But  this 
is  not  sufficient ;  the  geologist  must  leave  little  or  nothing  to 
chance  or  guess  work.  It  remains  to  correlate  the  facts  that 
have  been  collected  and  to  get  at  least  a  general  grasp  of  the 
stratigraphy  of  the  country  or  area  examined. 

This  is  not  a  matter  of  merely  academic  interest,  but  is  of 
very  practical  utility,  for  though  the  directions  of  variation 
may  be  known,  though  the  petroliferous  nature  of  the  series  be 
assured,  and  though  an  exceedingly  favourable  geological 
structure  be  discovered,  there  may  not  be  any  oil-bearing  rock 
of  importance  beneath  the  surface  within  reach  of  the  drill. 
Thus,  in  Lower  Burma  a  well  might  be  located  on  a  good  anti- 
clinal or  dome  structure,  among  what  used  to  be  known  as  the 
"Pfome  Series,"  which  is  undoubtedly  petroliferous,  and  on 
drilling  being  commenced  the  well  might  very  shortly  penetrate 
into  the  Sitshayan  shales,  a  marine  group  of  great  thickness 
which  has  never  yielded  petroleum.  Or  again,  in  either  Lower 
or  Upper  Burma  an  area  of  excellent  structure  high  up  in  the 
Pegu  Series  might  be  tested  where  the  depth  to  the  nearest 
petroliferous  bands  might  be  so  great  as  to  make  it  impossible 
to  reach  them,  or  if  possible,  at  an  expenditure  of  time  and 
money  that  would  effectually  prevent  the  field  from  being 
remunerative.  Instances  of  failure  under  such  conditions  are 
only  too  frequent,  and  similar  cases  can  be  mentioned  from 

121 


122  OIL-FINDING 

Trinidad,  Persia,  and  Baluchistan.  In  fact  the  writer,  even 
with  his  limited  experience  of  oilfield  exploitation,  has  come 
across  cases  of  failure  owing  to  neglect  of  stratigraphical  study 
in  every  field  with  which  he  is  intimately  acquainted. 

It  is  essential,  therefore,  that  the  main  stratigraphical 
groups  of  a  series  should  be  determined,  and  the  geologist  must 
be  able  to  recognize  within  reasonable  limits  the  position  in  the 
series  of  any  horizon  that  he  has  to  study. 

In  the  course  of  field-work  the  geologist  will  necessarily 
gather  a  great  number  of  facts  of  stratigraphical  importance, 
especially  during  his  study  of  the  lateral  variations,  for  it  is 
those  variations  which  complicate  the  issue,  and  make  the 
establishment  of  a  stratigraphical  sequence  a  matter  of  no  small 
difficulty.  A  correlation  or  tabulation  of  the  facts  is  necessary, 
and  as  each  new  area  or  district  is  examined  something  will  be 
found  to  add  to  or  modify  the  correlation  previously  attempted. 
Finality,  if  the  area  be  large,  is  almost  impossible  to  attain,  but 
the  broad  lines  may  be  laid  down  to  be  improved,  modified,  or 
confirmed  by  future  observers. 

Where  sections  through  the  entire  series  in  which  oil  occurs 
are  to  be  observed,  the  geologist  will  do  well  to  examine  them 
as  soon  as  possible ;  he  then  starts  with  a  sound  basis  for 
generalizations.  Measurements  of  the  thicknesses  of  groups  of 
different  types  of  sediment  should  be  made  wherever  possible 
and  noted  for  each  particular  district.  Such  measurements 
need  not  be  made  on  the  ground  if  evidence  be  abundant  and 
the  area  be  mapped  carefully  on  a  large  scale ;  sufficient 
accuracy  will  be  assured  by  measurements  on  the  map. 
Vertical  sections  of  the  strata  observed  should  then  be  con- 
structed for  each  district  or  locality. 

Lithological  characters  must  be  studied  closely,  but  too 
much  reliance  must  not  be  placed  on  them  for  purposes  of 
correlation ;  for  if  variation  be  rapid,  precisely  similar  conditions 
of  deposition  will  be  found  to  have  occurred  at  different  epochs 
in  different  areas,  and  may  occur  again  and  again  in  the  same 
area.  Thus  almost  any  particular  variety  of  strata  may  occur 
at  almost  any  horizon  in  a  thick  series,  and  to  found  any 
generalization  upon  resemblance  in  lithological  characters, 
unless  the  rocks  can  be  actually  traced  along  the  strike  from 
one  area  to  another,  may  lead  to  fatal  mistakes. 

State  of  Mineralization. — In  a  thick  series,  however,  there 


STRATIGRAPHY  123 

are  some  points  that  may  be  noted  with  great  advantage,  and  of 
these  first  of  all  comes  what  may  be  generally  expressed  as  the 
"  state  of  mineralization."  When  one  is  dealing  with  a  series 
of  from  5000  to  10,000  feet  of  strata— and  the  prospecting 
geologist  will  probably  have  to  study  a  mass  of  sediment 
somewhere  between  these  limits— it  is  only  natural  to  expect 
that  the  older  deposits  have  been  more  greatly  affected  than  the 
younger  by  the  conditions  of  temperature,  pressure,  and  circula- 
tion of  underground  waters  to  which  they  have  been  subjected, 
and  the  longer  period  during  which  these  conditions  obtained. 
Thus  harder  and  more  compact  strata  will  be  observed  among 
the  lower  horizons  than  among  the  upper,  even  when  the 
sediments  are  of  practically  the  same  composition.  Jointing, 
again,  will  be  more  perfectly  developed  in  the  older  strata, 
especially  in  the  argillaceous  rocks,  which  are  more  susceptible 
to  pressure  than  arenaceous  rocks.  Thus  a  concentric  weather- 
ing and  exfoliation  may  be  prevalent  among  clay  groups  in  the 
lower  part  of  a  series,  and  altogether  absent  from  similar  clays 
among  the  higher  horizons.  The  formation  of  veins,  whether  of 
selenite  or  calcite  in  clays,  and  the  slickensiding  of  these  veins 
owing  to  minute  movements,  is  another  point  to  be  noted. 
Ceteris  paribus,  these  are  always  more  conspicuous  among  the 
older  horizons.  If  the  series  contain  lignites  or  coals,  they  and 
their  underclays  usually  furnish  easily  recognizable  evidence, 
the  tendency  being  for  the  older  carbonaceous  deposits  to  have 
become  harder,  blacker,  better  jointed,  and,  as  proved  by 
analysis,  to  have  lost  water  to  a  greater  extent  than  the  younger, 
while  the  underclays  develop  at  least  the  rudiments  of 
stratification,  which  they  may  not  exhibit  till  subjected  to  con- 
siderable pressures. 

Among  arenaceous  strata  the  solution  of  iron  compounds  or 
calcium  carbonate,  and  their  redeposition  in  cementing  laminae, 
or  concentration  into  concretions,  are  effects  which  have  required 
time  as  well  as  the  necessary  conditions  as  regards  pressure, 
temperature,  and  presence  of  carbonated  water;  so  younger 
strata  may  give  very  little  evidence  of  such  action,  the  effects 
of  which  are  common  enough  in  strata  of  greater  age. 

All  these  minor  points,  none  of  which  is  of  great  importance 
in  itself,  may  by  their  cumulative  evidence  enable  the  field- 
student  to  detect  the  difference  between  a  lower  or  middle 
horizon  and  an  upper  horizon  in  the  series,  or  between  upper  or 


124  OIL-FINDING 

middle  and  lower  horizons,  so  that  in  dealing  with  a  consider- 
able thickness  of  strata,  exposed  in  an  inlier  and  perhaps  un- 
conformably  overlaid,  some  idea  may  be  at  once  suggested  as  to 
the  position  in  the  series  of  the  horizons  exposed.  It  must  be 
remembered  that  these  points  of  enquiry  are  of  chief  value  in 
Tertiary  strata,  where  the  youngest  rocks  are  very  little  altered 
since  their  deposition.  Where  folding  has  been  intense,  and  on 
a  large  scale,  the  mineralization  of  strata  has  naturally  pro- 
ceeded further  than  in  undisturbed  regions,  and  this  must  be 
taken  account  of  when  comparing  strata  from  different  areas. 

Alteration  in  Character  of  Sediment. — Frequently,  also, 
it  may  be  found  that  the  detritus  from  which  sediments  are 
formed  has  altered  in  character  as  the  series  is  ascended ; 
pebbles  of  some  particular  rock  may  be  found  in  the  upper  or 
lower  beds  alone ;  if  this  is  found  to  hold  good  over  a  wide  area 
it  becomes  of  great  importance  as  proving  that  different  strata 
were  being  denuded  at  different  times.  Thus  in  the  Yaw  sand- 
stones at  the  base  of  the  Pegu  Series,  pebbles  of  agate  are 
frequent  in  some  parts  of  Upper  Burma,  but  throughout  the 
rest  of  the  series  they  are  absent,  and  it  is  not  till  the  post- 
volcanic  stages  of  the  succeeding  and  unconformable  Irrawaddy 
Series  that  agate  pebbles  are  again  observed  in  the  Tertiary 
sediments.  Similar  instances  could  be  given  without  number, 
but  this  will  be  sufficient  to  illustrate  the  point  that  the  con- 
stituents of  an  arenaceous  group  may  on  occasion  furnish  a 
clue  to  its  age. 

Details  of  this  kind  may  be  noted  on  the  vertical  sections 
made  for  different  districts,  and  may  be  of  great  help  in 
establishing  the  stratigraphical  relations  of  different  groups. 

Fossil  Evidence. — Of  all  aids  to  correlation  and  the  working 
out  of  a  stratigraphical  sequence  that  will  hold  good  over  large 
areas,  there  is  nothing  more  valuable  than  fossil  evidence,  pro- 
vided that  it  is  abundant,  and  that  it  is  made  use  of  in  a 
practical  way,  as  the  handmaid  rather  than  the  mistress  of 
stratigraphy.  Let  no  practical  geologist  take  upon  himself  to 
despise  the  evidence  that  he  may  glean  from  fossil  fauna.  Their 
collection  and  study  may  entail  a  great  deal  of  extra  trouble, 
and  many  a  weary  day  spent  indoors,  but  any  definite  results 
obtained  are  certain,  and  may  enable  correlations  to  be  made 
that  cannot  be  accomplished  by  any  other  means. 


STRATIGRAPHY  125 

The  writer  confesses  to  have  little  patience  with  the 
zoological  side  of  palaeontology,  and  even  to  be  indifferent  as 
to  the  name  that  may  be  given  to  the  fossil  part  of  any  particular 
organism,  but  he  has  had  experience  of  what  can  be  done  in 
the  way  of  correlating  isolated  and  far-separated  areas  by  the 
careful  mapping  of  fossiliferous  horizons  and  the  collection  of 
their  fossil  organisms,  even  where  faunal  change  in  time  is  slow 
and  the  species  many  and  often  ill-preserved. 

Even  if  the  field-student  be  not  interested  in  palseontological 
work,  even  if  he  be  ignorant  of  the  generic  names  of  the 
organisms,  he  will  do  well  to  collect  and  label  them  carefully, 
and  note  on  his  vertical  sections  the  horizons  from  which  they 
were  obtained.  Let  him  call  them  "  Tom,  Dick,  and  Harry  " 
if  he  will,  so  long  as  he  can  recognize  them  again  and  can 
point  to  the  horizons  from  which  they  were  collected. 

It  may  be  of  interest,  and  of  use  also,  to  the  petroleum 
geologist  if  a  brief  description  is  given  here  of  the  methods  of 
handling  palseontological  evidence,  originated  and  put  in  practice 
by  the  Burmah  Oil  Company's  Geological  Staff. 

In  Burma  one  of  the  chief  difficulties  is  in  the  correlation 
of  different  fields,  as  the  petroliferous  Pegu  Series  appears 
frequently  in  widely  separated  inliers.  These  inliers  are 
overlaid  unconformably  by  the  fluviatile  Irrawaddy  Series, 
during  or  previous  to  the  deposition  of  which  there  was 
extensive  and  sometimes  very  great  denudation  of  the  under- 
lying strata.  Thus  the  local  base  of  the  Irrawaddy  Series  may 
be  found  resting  upon  almost  any  horizon  in  the  Pegu  Series, 
and  measurements  downwards  from  the  base  of  the  upper  series 
are  useless  as  an  aid  to  the  determination  of  horizons  in  the 
Pegu  strata  as  a  whole.  The  amount  of  pre-Irrawaddy 
denudation  varies  greatly  within  short  distances.  Added  to 
this  there  is  a  lateral  variation  in  the  Pegu  Series  so  great 
that  correlation  of  areas  by  a  study  of  lithological  characters 
is  practically  impossible,  unless  the  areas  are  close  together, 
while  the  main  groups  of  strata  in  any  field  thicken  and  thin 
out  with  bewildering  rapidity,  so  that  the  whole  series,  even 
where  the  upper  part  is  not  removed  by  denudation,  varies 
greatly  in  thickness  in  different  districts. 

The  area  examined  by  the  Geological  Staff*  of  the  Burmah 
Oil  Company  up  to  date  is  approximately  20,000  square  miles, 
most  of  it,  however,  covered  by  younger  deposits  than  the 


126  OIL-FINDING 

petroliferous  series.  The  Pegu  Series  at  its  greatest  development 
reaches  a  thickness  of  at  least  10,000  feet.  It  will  be  readily 
understood  that  the  problem  of  working  out  the  stratigraphy,  so 
that  the  thickness  of  strata  containing  petroliferous  beds  should 
be  known  in  each  district,  and  the  horizons  exposed  in  each 
inlier  identified,  presented  many  difficulties. 

Luckily  there  was  a  considerable  mass  of  evidence  from  boring 
journals  available,  but  it  would  have  been  of  little  value  without 
palseontological  evidence,  which  is  abundant,  almost  every  inlier 
containing  at  least  one  and  generally  two  or  three  rich  faunas. 

Some  means  had  to  be  devised  to  enable  correlations  of 
isolated  areas  to  be  made,  and  palseontological  evidence,  if 
available  in  sufficient  quantity,  was  obviously  suggested.  It  was 
ten  days  of  continuous  rain  when  encamped  in  a  very  fossili- 
ferous  district  that  first  turned  the  writer's  thoughts  towards  an 
enquiry  as  to  whether  there  was  sufficient  f  aunal  change  through 
the  Pegu  Series  to  allow  of  its  being  subdivided  into  zones. 

Dr.  Noetling  of  the  Indian  Geological  Survey  had  published 
seven  years  previously  a  memoir  ("  Palaeontographica  Indica," 
A7ol.  I.)  on  the  fauna  of  the  Burmese  Tertiary  rocks,  describing 
and  figuring  two  hundred  and  eight  species,  and  attempting  a 
stratigraphical   arrangement  of  the   sections  examined  up  to 
that   time.      Comparatively  little   of  Burma   had   been   gone 
over  by  then,  and  many  of  the  fossil  collections  were  made  by 
previous    observers,   the    localities   from  which   some  of  the 
faunas  were  obtained  were  but  vaguely  known,  and  the  relative 
positions  of  different  beds  in  the  series  were  uncertain.     Thus 
in   spite  of  the  ability  with  which    Dr.  Noetling  marshalled 
the  evidence,  he  was  handicapped  at  the  start  by  mistakes  in 
stratigraphy  inevitable  when  no  large-scale  mapping  is  done. 
The  mistakes  were  also  made  of  referring  every  section  examined 
in  Burma  to  a  type-section  in  the  Prome  District,  of  arbitrarily 
subdividing  the  series  into  a  supposed  fossiliferous  and  non- 
petroliferous  upper  division  and  a  petroliferous  and  non-fos- 
siliferous  lower  division,  and  of  treating  the  local  faunas  as 
"  zones."  Every  geological  observer  who  has  done  work  in  Burma 
since  the  publication  of  Dr.  Noetling's  Memoir,  and  has  started 
with  it  as  a  basis,  has  helped  to  bring  about  almost  inextricable 
confusion,  from  the  effects  of  which  the  official  work  of  the  Indian 
Geological  Survey  in  the  Burmese  Tertiaries  is  only  beginning 
to  emerge. 


STRATIGRAPHY 


127 


The  Geological  Staff  of  the  Burmah  Oil  Company  when 
driven  perforce  to  study  fossil  evidence  in  the  attempt  to 
correlate  different  areas,  began  by  making  vertical  sections  of 
each  field  surveyed,  marking  the  horizons  of  each  fossiliferous 
bed  and  each  oil-bearing  band.  Mapping  was  usually  done  on 
the  six-inch,  but  occasionally  on  the  eight-inch  scale,  so  it 
was  possible  to  make  fairly  accurate  estimates  of  the  thicknesses 
of  strata  exposed.  The  faunas  collected  were  then  compared  with 
Noetling's  faunas  treated  as  if  they  were  "  zones,"  but  arranged 
in  a  somewhat  different  order  from  that  published  in  the 
"  Palseontographica  Indica,"  as  it  very  soon  became  apparent 
that  some  modification  in  his  stratigraphical  arrangement  would 
have  to  be  made. 

Areas  where  many  fossiliferous  beds  at  different  horizons 
were  found  soon  demonstrated  that  there  is  considerable  faunal 
change  in  time  relation  in  the  Pegu  Series,  and  a  number  of 
rough  graphs  or  diagrams  were  made  use  of  to  illustrate  this, 
still  using  Dr.  Noetling's  "  zones  "  as  a  basis.  The  "  zones  " 


FIG.  9. 


FIG.  10. 


A,  B,  C,  etc.  were  arranged  vertically,  and  distances  Aa,  B6, 
etc.  were  measured  off  horizontally  in  proportion  to  the  number 
of  species  common  to  the  zone  in  each  bed  (Fig.  9). 

Joining  the  ends  of  these  horizontal  lines  we  get  a  figure 
dbcdef,  which  in  the  case  illustrated  indicates  that  the  fauna  of 
the  bed  under  examination  resembles  the  fauna  of  zone  C  most 
closely,  but  is  probably  somewhat  higher  in  the  series.  The 


128  OIL-FINDING 

method  is  rough  and  open  to  many  objections,  but  if  the  zones 
contain  a  sufficient  number  of  species,  and  the  fauna  to  be 
examined  is  a  rich  one,  an  ambiguous  diagram  such  as  Fig.  10, 
which  leaves  it  doubtful  whether  the  bed  should  be  placed  near 
the  top  or  the  base  of  the  series,  is  hardly  possible.  Another 
advantage  is  that  the  breadth  of  the  diagram  shows  at  once  if 
the  fauna  be  a  rich  one  or  not;  the  greater  the  number  of 
species  in  a  bed,  the  more  weight  is  naturally  given  to  the 
evidence  obtained  from  it.  . 

Many  difficulties  had  to  be  encountered.  For  instance,  the 
discovery  of  species  not  described  by  Noetling  tended  from  the 
first  to  complicate  matters.  This  difficulty  was  partially  got 
over  by  procuring  such  books  of  reference  as  were  available : 
Dr.  Martin's  beautiful  figures  and  descriptions  in  "  Die  Fossilen 
von  Java  "  proved  of  great  assistance. 

Then  it  was  found  that  Dr.  Noetling's  zones  did  not  make  a 
satisfactory  basis,  some  of  his  faunas  are  littoral,  some  laminarian 
or  pelagic,  and  some  indicate  brackish  water  conditions ;  some 
are  very  rich  in  gasteropods  with  few  lamellibranchs,  and 
others  contain  numbers  of  lamellibranchs  while  gasteropods  are 
poorly  represented.  It  became  evident  that  true  zones  were 
required,  not  faunas  from  single  beds. 

Several  sections  were  mapped  and  measured  from  the  base 
to  the  top  of  the  Pegu  Series,  and  all  fossiliferous  beds  in  them 
carefully  collected.  It  was  possible  then,  by  a  comparison  of 
the  vertical  sections,  to  combine  them  and  place  many  of  the 
faunas  in  their  true  relative  positions.  The  measurements  of 
the  thicknesses  of  groups  were  of  great  use  in  this,  though 
owing  to  the  thinning  and  thickening  of  different  parts  of  the 
series  no  fossil  bed  could  be  placed  in  the  series  entirely  on 
such  evidence,  unless  it  was  confirmed  by  palseontological  data. 

A  range- table  was  tentatively  constructed ;  it  contained 
between  280  and  300  species  arranged  horizontally,  while  the 
horizons  were  arranged  vertically.  A  small  diagonal  cross 
marked  each  occurrence  of  a  species,  and  every  occurrence  of 
the  same  species  lay  upon  a  vertical  line  indicating  the  range 
of  the  form  so  far  as  it  has  been  ascertained.  The  majority  of 
the  forms  dealt  with  were  gasteropods,  which  were  not  only 
more  easily  identified  than  the  lamellibranchs,  but  which 
seemed  to  show  more  marked  changes  in  time  relation.  The 
lamellibranchs,  however,  proved  of  great  value,  though  the 


STRATIGRAPHY  129 

ranges  of  some  forms  are  apparently  very  long.  Echinoderms, 
Crustacea,  and  corals  proved  very  useful,  but  they  do  not 
always  occur  in  sufficient  numbers  in  the  littoral  beds  which  are 
usually  the  most  fossiliferous  members  of  the  Pegu  Series. 
Fish,  mammals,  foraminifera,  and  one  solitary  brachiopod  have 
been  made  use  of ;  in  fact,  the  occurrence  of  every  organism 
found  was  recorded  on  the  range-table. 

Dr.  Noetling's  faunas  were  made  use  of  after  being  placed 
among  the  faunas  as  nearly  as  could  be  ascertained  in  their  true 
stratigraphical  positions. 

After  the  range-table  was  constructed,  it  was  divided 
horizontally  into  seven  zones  by  means  of  horizontal  lines  at 
convenient  intervals. 

When  any  new  fauna  was  discovered,  and  the  species 
identified,  it  was  compared  with  the  faunas  of  the  several  zones 
by  means  of  the  graphs  or  diagrams  mentioned  above.  There 
was  very  seldom  any  doubt  as  to  the  zone  to  which  a  fauna  natur- 
ally belonged.  Having  ascertained  the  zone,  the  fauna  was 
then  compared  with  the  chief  faunas  in  the  zone  and  its  relative 
position  towards  them  ascertained,  and  if  there  was  no  doubt  as 
to  its  position  the  new  fauna  was  at  once  put  upon  the  range- 
table.  Comparison  of  vertical  sections  was  of  great  assistance 
in  the  placing  of  a  new  fauna. 

The  first  "range-table  served  very  well,  but  it  soon  became 
out  of  date.  Faunas  from  all  parts  of  Burma  were  constantly 
being  brought  in,  and  great  numbers  of  entirely  new  species 
came  to  light.  In  fact,  evidence  accumulated  so  rapidly  that 
additions  and  modifications  were  constantly  being  made.  The 
attempt  to  reconcile  carefully-measured  sections  with  Dr. 
Noetling's  stratigraphical  arrangements  proved  a  matter  of 
difficulty,  and  after  the  same  sections  that  he  describes  had 
been  carefully  mapped,  new  vertical  sections  were  substituted 
for  his,  and  it  was  decided  that  when  a  new  range-table  had 
to  be  constructed  Dr.  Noetling's  faunas  should  be  omitted, 
and  the  stratigraphical  arrangement  of  fossils  based  upon  the 
four  or  five  complete  sections  from  base  to  the  top  of  the  series 
which  were  available. 

The  new  range-table  deals  with  approximately  one  hundred 
rich  faunas  and  numberless  beds  containing  only  a  few  species. 
The  number  of  species  and  distinct  sub-species  or  variations  is 
between  450  and  500.  The  work,  the  magnitude  of  which  only 

K 


130  OIL-FINDING 

gradually  became  apparent,  can  perhaps  never  be  complete ;  the 
collection  in  the  Company's  geological  office  numbers  many 
thousands  of  specimens,  nearly  all  of  which  are  in  a  good  state 
of  preservation.  All  doubtful  identifications  have  been  rejected 
and  are  not  entered  on  the  range- table. 

The  length  of  range  of  many  of  the  forms,  and  these  often 
among  the  commonest,  has  proved  disappointing,  but  in  this 
the  accumulation  of  material  has  been  of  benefit,  as  with  more 
and  better  specimens  it  is  often  possible  to  detect  variations 
and  to  split  a  species  into  two  sub-species,  one  being  character- 
istic of  the  lower  part  of  the  series  and  one  of  the  upper. 

The  occurrence  of  certain  types  in  certain  provinces  and 
apparently  not  in  others  was  one  of  the  initial  difficulties,  but 
this  has  gradually  yielded  to  the  effect  of  more  and  more 
evidence  being  brought  forward :  the  fauna  of  the  Pegu  Sea  at 
any  epoch  seems  to  have  been  fairly  constant  over  the  area  in 
which  it  has  been  studied. 

The  series  is  now  divided  into  five  main  zones,  and  further 
subdivision  is  possible.  Though  seldom  more  than  sixteen  or 
twenty  species  are  found  only  in  one  zone,  the  ranges  of  many 
of  the  forms  are  sufficiently  short  to  be  of  great  stratigraphical 
value.  Any  mixed  fauna  of  twenty  to  thirty  species  can  usually 
be  placed  without  any  difficulty  in  its  true  stratigraphical 
position,  and  a  difference  of  200  feet  in  horizon  between  two 
rich  faunas  can  be  shown  by  diagrams.  The  collection  of  fifty 
species  from  one  bed  is  by  no  means  a  rare  occurrence  in 
Burma. 

The  methods  employed  in  dealing  with  such  a  mass  of 
palaeontological  evidence  are  at  the  best  rough  and  ready,  and 
may  not  commend  themselves  to  palaeontologists  generally,  but 
the  practical  use  of  fossil  evidence  for  practical  purposes  con- 
nected with  oil  development  has  been  the  point  always  aimed 
at.  Many  species  have  no  doubt  been  incorrectly  named,  many 
even  of  the  genera  may  not  have  been  determined  beyond  the 
possibility  of  doubt,  but  the  ten  thousand  feet  of  the  Pegu  Series 
have  been  divided  into  zones  which  have  held  good  up  to  the 
present,  the  effects  of  lateral  variation  have  been  abundantly 
proved,  the  advance  of  the  delta  has  been  determined  beyond 
the  possibility  of  doubt ;  it  is  possible  to  state  with  a  fair  degree 
of  accuracy  what  zones  in  each  district  will  be  petroliferous  and 
and  what  zones  barren,  and  every  bed  with  a  rich  fauna  can  be 


STRATIGRAPHY  131 

placed  in  the  series  within  one  or  two  hundred  feet  of  its  true 
position,  and  datum  lines  for  the  correlation  of  any  new  field 
are  furnished.  Many  details  also  of  the  geology  of  Burma, 
details  of  which  it  would  not  be  in  the  interests  of  the  Burmah 
Oil  Company  to  permit  the  publication  at  present,  have  been 
brought  to  light. 

It  is  not  intended  that  this  palaeontological  work  of  the 
Burmah  Oil  Company's  Geological  Staff  should  be  taken  as  an 
object  lesson  by  the  field-student ;  this  brief  account  of  it  has 
been  given  merely  to  show  what  practical  value  can  be  derived 
from  the  study  of  fossils  by  a  staff,  none  of  whom  would  claim 
to  be  specially  qualified  as  a  palaeontologist.  It  is  urged  upon 
the  geologist  who  is  engaged  in  oilfield  work  to  collect  such 
fossils  as  he  may  find,  and  to  label  them  carefully  for  future 
study.  They  may  prove  of  great  assistance  at  some  future 
day,  although  apparently  of  little  interest  or  importance  at  the 
time  that  they  were  collected.  So  long  as  palaeontology  is  kept 
in  its  proper  relation  to  field-mapping,  so  long  as  generalizations 
are  not  founded  upon  the  sporadic  occurrence  of  a  few  species* 
but  on  evidence  from  a  large  thickness  of  strata  and  a  wide 
range  of  organisms,  every  fact  that  can  be  brought  to  light  and 
tabulated  will  be  of  service. 

The  idea  of  a  range -table  is  to  safe-guard  against  sources 
of  error,  for  a  few  wrong  identifications  from  a  bed  containing 
many  species  may  not  affect  the  final  result  appreciably.  The 
more  species  collected  and  identified  from  a  bed,  the  more 
certainty  will  be  attained  in  assigning  it  to  its  stratigraphical 
horizon.  It  is  a  very  old  proposition,  but  never  more  aptly 
illustrated  than  in  palaeontological  work,  that  a  correct  con- 
clusion is  more  easily  reached  by  considering  a  great  number 
of  minor  points,  none  of  which  may  be  in  itself  of  supreme 
importance,  but  the  cumulative  effect  of  which  is  great,  than 
by  seizing  upon  three  or  four  salient  facts  and  founding  a 
generalization  upon  them. 


CHAPTER  VIII 
LOCATION   OF   WELLS 

ANY  practical  operator,  manager,  field-superintendent  or  driller, 
who  has  been  good-natured  enough  to  read  so  far  in  this  little 
book  may  well  exclaim  "  Now  at  last  we  come  to  practical 
politics :  what  can  this  geologist  tell  us  about  the  locating  of 
wells  ? "  With  the  spirit  of  such  a  reader  the  writer  heartily 
agrees.  All  that  has  gone  before  is  leading  up  to  this  one 
important  matter,  the  choosing  of  the  sites  for  oil  wells,  so  that 
the  oil-bearing  strata  may  be  struck  with  a  minimum  of  trouble 
and  expense,  and  under  conditions  that  should  yield  a  maximum 
production. 

It  is,  of  course,  in  the  case  of  the  first  test-well  of  a  new 
field,  or  presumed  field,  that  the  importance  of  carefully  select- 
ing a  site  is  most  forcibly  brought  home  to  us,  and  it  is  this 
aspect  also  which  appeals  most  to  the  general  public.  The 
geologist  who  undertakes  oilfield  work  will  soon  weary  of  the 
oft  reiterated  question,  "  How  do  you  know  where  to  put  a 
well  ? " 

There  are  many  methods  of  actually  making  a  first  selection. 
It  is  told  of  one  well-known  and  very  successful  exploiter  and 
driller  in  the  United  States  that  he  frankly  stated  that  his 
method  was  to  put  on  an  old  and  cherished  hat,  and  to  gallop 
a  rough  horse  about  the  country  side  or  farm  till  the  hat 
dropped  off.  On  the  spot  where  it  fell  he  drilled  the  well. 
The  story  is  at  least  ben  trovato,  and  it  is  possibly  quite 
true. 

The  writer  knows  one  highly  productive  and  very  valuable 
field,  miles  from  the  nearest  surface  indication,  where  the  first 
test-well  site  was  selected  in  almost  as  haphazard  a  fashion. 
Drillers  and  field- superintendents  had  met  to  make  the  location, 
and  the  area  in  which  a  spot  was  to  be  selected  was  generally 
determined,  but  with  characteristic  caution  none  would  venture 

132 


LOCATION    OF   WELLS  133 

an  opinion  before  the  others  as  to  what  exact  spot  should  be 
fixed  upon.  At  last,  one  bolder  spirit  than  the  others,  spoke 
up  and  said,  "  Well,  boys,  if  it's  all  the  same  to  you,  let's  put 
the  well  where  that  crow  sits  down,"  pointing  at  the  same 
time  to  a  crow  which  was  flying  about  them.  The  crow 
alighted,  the  spot  was  marked,  and  the  well  drilled  with 
remarkably  successful  results ;  it  is  still  producing  after 
eleven  years.  A  flight  of  a  hundred  yards  or  so  further  to 
the  eastward  would  have  put  the  well  beyond  any  hope  of 
striking  oil. 

Well-sites,  in  fact,  have  been  selected  for  very  many 
reasons ;  the  colour  of  the  soil,  or  the  proximity  to  an  oilshow 
have  frequently  been  responsible  for  the  erection  of  a  derrick 
at  a  particular  spot.  The  divining  rod  has  been  utilized 
occasionally,  and  sometimes  with  successful  results,  while 
complicated  instruments  have  been  invented  and  put  on  the 
market  to  enable  any  one  to  detect  the  presence  of  oil  beneath 
the  surface,  but  as  to  whether  or  no  there  is  any  scientific 
basis  for  the  working  of  such  instruments  the  author  must 
plead  ignorance. 

But  the  ordinary  workaday  geologist  must  not  depend  on 
quasi- supernatural  aids  nor  little  understood  inherited  instincts. 
By  his  geological  map  he  must  stand  or  fall,  for  he  will  soon 
appreciate  the  fact  that,  however  good  and  careful  his  work 
may  be,  it  is  upon  the  wells  that  he  locates,  especially  in  new 
fields,  and  upon  the  results  obtained  from  them,  that  he  will 
be  judged.  An  error  of  judgment  made,  a  fact  lost  sight  of, 
a  calculation  not  checked  and  rechecked,  an  allowance  not 
made  for  some  condition  that  may  be  inferred  but  cannot  be 
observed,  and  the  well  may  prove  a  failure,  with  the  effect  that 
his  reputation  as  a  practical  man  may  suffer  undeservedly. 

The  popular  idea  that  petroleum  is  a  very  capricious  and 
uncertain  mineral,  and  that  the  only  way  to  be  sure  of  finding 
it  is  to  drill  a  borehole,  is  rapidly  dying  out,  but  still  it  is  not 
possible  to  drill  for  oil  with  the  same  confidence  with  which 
one  can  drill  for  water.  It  is  often  impossible  to  be  sure 
whether  there  is  petroleum  beneath  the  surface  or  not,  but 
fortunately  it  often  is  possible  to  be  quite  certain  that  oil  will 
not  be  obtained  by  drilling. 

When  the  series  has  been  proved  to  be  petroliferous  in  the 
particular  district,  when  the  stratigraphy  has  been  worked  out, 


134  OIL-FINDING 

and  it  is  known  that  oil-bearing  horizons  are  within  reach  of 
the  drill,  and  when  the  geological  structure  has  been  proved  to 
be  favourable,  the  striking  of  oil  becomes  almost  a  matter  of 
certainty.  In  such  a  case,  when  the  geological  map  on  a  large 
scale  has  been  completed,  the  locality  for  the  first  test-well  is 
indicated  beyond  a  doubt,  and  it  only  remains  to  select  the 
most  convenient  spot  for  access,  transport,  water  supply,  etc. 
within  that  locality. 

But  though  the  map  shows  the  exact  spot  that  should  give 
the  most  favourable  results,  it  will  be  the  task  of  the  geologist 
to  find  that  spot  on  tJie  ground,  and  to  see  that  no  mistake  is 
made  in  marking  it.  This  is  the  more  important  as  the 
first  geological  survey,  though  entirely  correct  as  regards 
structure,  may  not  be  accurate  as  regards  topography ;  the  well 
must  therefore  be  located  according  to  the  geological  structure 
rather  than  according  to  the  topography. 

In  symmetrical  domes  and  anticlines  a  position  upon  the 
highest  point  of  the  crest  is  indicated,  that  is  to  say,  where  the 
crest  reaches  a  maximum  height  geologically  speaking,  and 
probably  quite  independent  of  the  surface  contour  of  the  ground. 
Towards  such  a  locality  oil  and  gas  will  naturally  tend  to 
migrate,  gas  pressure  should  be  highest,  and  production  greatest. 

But  even  in  this  case  it  may  be  -advisable  to  select  a  site 
slightly  removed  from  that  theoretically  indicated,  for  purely 
practical  reasons.  Where  gas-pressure  is  likely  to  be  very 
great  and  the  structure  is  favourable  to  a  great  concentration 
towards  a  crestal  point,  a  well  might  encounter  great  difficulty 
by  striking  a  violent  discharge  of  gas  before  any  oil  is  reached, 
and  it  might  be  necessary  to  allow  the  gas  to  blow  off  for 
months  before  drilling  could  be  continued  into  the  oilrocks, 
and  oil  produced  in  any  quantity.  If  the  gas  could  be  con- 
trolled and  utilized  at  once,  there  could  be  little  objection  to 
the  drilling  of  such  a  well,  but  this  might  not  be  possible  in 
the  circumstances,  and  much  valuable  gas-pressure  might  be 
dissipated  before  the  well  could  be  brought  in.  In  such  cases 
the  well  often  drills  itself  in,  but  this  is  seldom  a  satisfactory 
result,  as  the  handling  and  anchoring  of  casing  may  be  pre- 
vented by  the  rush  of  gas  and  oil.  A  well,  however,  located 
slightly  down  the  pitch  or  the  flank  of  the  flexure  is  not  so 
likely  to  meet  with  the  same  difficulty,  but  will  probably  be  a 
producing  ^oilwell  ^as  soon  as  the  oilrock  is  reached,  so  that 


LOCATION   OF   WELLS  135 

a  much  better  idea  of  the  capabilities  of  the  field  will  be 
obtained  without  delay. 

It  is  possible  that  in  very  few  cases  is  gas  stored  at  the 
crest  of  an  anticline  to  the  exclusion  of  oil,  but  it  is  quite 
probable  that  gas  may  be  struck  before  the  oilrock  is  reached, 
and  the  pressure  may  be  great  enough  to  cause  damage  to 
plant,  if  not  even  loss  of  life,  when  a  well  suddenly  taps  such 
an  accumulation  of  gas  under  very  high  pressure. 

In  those  cases  where  large  mud-volcanoes  occur  on  the 
crests  of  anticlines,  similar  precautions  must  be  taken  in  locating 
the  first  well.  High  gas-pressure  and  possibly  flows  of  mud 
may  make  the  drilling  very  difficult,  if  not  impossible,  on  the 
crest  of  the  flexure,  while  a  well  slightly  down  the  flank  may 
not  suffer  from  the  same  disadvantage.  The  geologist  must 
judge  from  the  results  of  wells  drilled  under  similar  conditions 
in  the  same  country,  or  from  his  experience  in  other  countries, 
whether  there  be  any  danger  of  a  well  proving  troublesome  in 
this  manner. 

When  the  dome  or  anticline  is  asymmetrical  the  well  must 
be  placed  not  on  the  crest  but  on  the  flank  on  which  the  gentler 
clips  occur.  This  is  owing  to  the  hade  of  axial  plane  of  the 
flexure,  and  the  reason  is  obvious  when  a  horizontal  section 
through  the  fold  is  drawn  to  scale.  Mr.  E.  H.  Pascoe  has 
explained  this  point  very  clearly  in  the  "  Records  of  the  Indian 
Geological  Survey,"  Vol.  XXXIV.,  Part  iv.,  1906,  where  he 
gives  a  formula  by  which  the  distance  from  the  crest  at  which 
a  well  should  be  located  on  an  asymmetrical  anticline  can  be 
calculated,  as  follows  : — 

I  =  d  tan  0  +  X 

where  I  is  the  distance  from  the  crest  to  the  well,  d  is  the  depth 
of  the  well,  0  is  the  angle  of  hade  of  a  plane  through  the  apex 
of  the  fold,  and  X  is  the  distance  between  what  he  calls  the 
"  apex- locus  "  and  the  "  crest-locus."  Thus  a  well  at  A  (Fig. 
11)  will  just  touch  oil  in  the  petroliferous  bed  1,  while  a  well  at 
D  will  strike  oil  in  the  oilrocks-  3  and  4,  but  not  in  the  higher 
beds  1  and  2.  The  angle  0  must  be  found  by  observation :  it 
is  half  the  difference  between  the  steepest  dips  observed  on 
either  side  of  the  crest  in  the  same  led.  Thus,  if  the  maximum 
dip  of  a  bed  on  one  flank  is  90°,  and  on  the  other  flank  10°,  9 

will  be  90°  I  10°  =  40°. 


136 


OIL-FINDING 


This  formula  is  of  great  practical  value  in  determining  the 
best  position  for  a  well,  when  some  evidence  is  to  hand  from 
the  other  wells  in  the  vicinity.  It  leaves,  however,  several 
details  to  be  worked  out  practically.  Thus,  unless  the  depth  d 
to  be  drilled  is  known  approximately,  it  is  impossible  to  find  a 


FIG.  11. — Section  of  Asymmetrical  Anticline.    Dotted  portic 
extent  of  oil-impregnation. 


shows 


value  for  /.  Again,  the  distance  X,  which  will  also  vary  accord- 
ing to  the  depth,  must  be  found  by  calculation  ;  it  depends  on 
the  shape  and  sharpness  of  the  flexure,  which  may  vary  greatly 
in  different  localities.  It  should  be  possible  to  calculate  X  from 
observation,  when  the  strata  are  well  exposed.  It  will  almost 
certainly  decrease  gradually  as  lower  and  lower  horizons  are 
reached. 

But  the  calculation  of  d  is  a  matter  of  greater  difficulty, 
unless  evidence  from  other  wells  is  available,  or  the  oil-bearing 
horizons  are  known  through  very  careful  stratigraphical  work. 
Thus  in  a  new  field  that  is  to  be  tested  it  will  be  expedient  to 
place  the  test-well  so  that  the  crest  will  not  be  crossed  at  the 
greatest  depth  to  which  it  is  proposed  to  drill.  This  may 
necessitate  the  missing  of  the  oilpools  at  shallow  depths,  so 


LOCATION    OF   WELLS 


137 


the  geologist  must  consider  each  case  on  its  merits  and  locate 
his  well  for  deep  or  shallow  oilrocks  as  is  most  convenient  or 
most  likely  to  prove  of  value  to  the  company  developing  the 
area.  As  a  general  rule,  it  will  be  found  better  to  exploit  the 
shallow  sands  first,  once  the  presence  of  oil  has  been  proved, 
stating  the  depth  to  which  each  well  is  to  be  drilled,  while 
another  well  can  be  located  further  from  the  crest  to  test  deep 
oilrocks. 

One  point  with  regard  to  asymmetrical  anticlines  and  domes 
seems  to  have  been  lost  sight  of  very  frequently,  and  that  is, 


FIG.  12.— Asymmetrical  Anticline,  showing  decrease  in  Hade  of  Axis. 
Dotted  portion  shows  extent  of  oil-impregnation. 

that  the  hade  of  the  axial  plane  in  any  flexure  is  not  constant. 
Tracing  the  axis  along  the  flexure,  the  hade  is  seen  to  decrease 
or  increase,  but  it  is  not  so  readily  admitted  that  traced  down- 
wards into  the  heart  of  the  fold  the  hade  must  decrease.  Yet 
when  we  consider  that  the  flexure  has  been  caused  by  tangen- 
tial stress,  it  is  obvious  that  the  hade  of  any  asymmetrical 
flexure  must  decrease  downwards,  and  finally  disappear  alto- 
gether. In  Fig.  12,  which  represents  an  asymmetrical  fold  on 


138  OIL-FINDING 

a  scale  sufficiently  small  to  include  practically  the  whole  of 
the  flexure,  it  will  be  seen  that  the  hade  of  the  axial  plane 
decreases  rapidly,  and  that  to  calculate  on  its  remaining  con- 
stant and  to  drill  on  that  theory  would  be  to  court  failure. 
Again,  it  will  be  seen  that  at  a  certain  distance  from  the  crest 
the  hade  beneath  the  surface  has  practically  died  out ;  no  well 
need  therefore  be  drilled  further  from  the  crest  than  this  point, 
the  approximate  position  of  which  can  usually  be  arrived  at  by 
a  careful  study  of  sections  taken  across  the  whole  flexure.  No 
formula  can  be  given  for  finding  the  distance  of  this  point  from 
the  crest,  but  as  it  will  be  a  suitable  place  for  all  wells  greater 
than  a  certain  depth,  it  should  be  the  geologist's  endeavour  to 
ascertain  the  position  of  the  point  as  accurately  as  possible  and 
to  locate  the  deep  test  of  the  field  upon  the  line  indicated. 

Another  point  is  illustrated  in  the  section.  It  will  be  seen 
that  in  a  well  which  has  been  placed  too  far  from  the  crest,  a 
thin  bed  of  porous  rock  has  been  pierced  almost  on  a  level  with 
a  thicker  oil-bearing  band  which  forms  the  crest  of  the  fold. 
It  is  not  an  unfrequent  phenomenon  to  encounter  a  show  of  gas 
and  filtered  oil  in  a  thin  and  probably  lenticular  band  placed 
in  such  a  position.  The  oil  which  it  contains  has  come  from 
one  of  the  main  oilrocks,  and  has  been  filtered  during  its  migra- 
tion. It  is  struck  at  a  depth  much  less  than  that  calculated 
for  the  main  oilrock  that  the  well  has  been  drilled  to  strike. 
The  occurrence  of  this  show  of  filtered  oil  is  very  naturally 
regarded  as  a  hopeful  indication,  and  the  well  may  be  continued 
to  a  great  depth,  though  owing  to  the  decrease  of  the  hade  of 
the  axial  plane  it  is  impossible  to  strike  oil  in  the  main  oilrock. 
Instances  of  this  have  come  under  the  writer's  knowledge  more 
than  once,  and  till  the  whole  underground  structure  of  the  field 
has  been  clearly  proved,  it  may  be  impossible  to  satisfy  oneself, 
or  those  responsible  for  the  exploitation  of  the  field,  that  the 
show  of  filtered  oil  need  not  be  a  hopeful  indication  at  all. 

These  considerations  apply  chiefly  to  highly  asymmetrical 
anticlines,  where  the  flexuring  is  sharp.  In  flexures  that  are 
only  slightly  asymmetrical  and  are  not  very  sharp,  it  matters 
very  little  on  which  side  of  the  crest  the  well  is  drilled.  It  is 
possible  to  obtain  oil  in  quantity  from  the  steeply- dipping  flank 
of  an  anticline,  but  it  is  obvious  that  the  reservoir  on  that  side 
can  never  be  so  large  as  on  the  more  gently-dipping  flank. 
The  migration  of  petroleum  towards  the  crest  is  also  a  simpler 


LOCATION    OF   WELLS  139 

matter  in  steeply-dipping  beds,  and  a  well-defined  water  level 
may  quite  possibly  be  found  on  the  steeper  flank  considerably 
above  the  water  level  on  the  gentler  flank.  This  is  due  to  the 
hydrostatic  pressure  of  water,  which,  according  to  theory, 
probably  underlies  the  oil ;  in  steeply-dipping  beds  the  separa- 
tion of  oil  and  water  by  migratory  movements  along  or  up  the 
bedding  planes  is  naturally  favoured  more  than  in  gently- 
inclined  beds.  Accordingly,  it  is  always  as  well  to  make 
locations  on  the  gently-clipping  flanks  of  anticlines  whenever 
they  show  asymmetry.  Another  reason  of  a  practical  kind 
emphasizes  this  desirability ;  the  mechanical  difficulties  of  dril- 
ling through  steeply-dipping  strata  are,  in  most  cases,  much 
greater  than  when  the  strata  are  gently  inclined,  and  the 
tendency  of  the  bore  to  depart  from  the  vertical  position,  and 
for  the  sides  of  the  borehole  to  cave,  are  always  greater,  the 
more  steeply  the  strata  are  dipping. 

The  asymmetry  of  a  fold  very  frequently  changes  when  it  is 
traced  some  distance,  and  the  actual  hade  of  the  axial  plane 
may  change  from  one  side  to  the  other ;  hence  in  every  locality 
the  amount  and  direction  of  hade  must  be  ascertained  as  care- 
fully as  possible  and  locations  made  according  to  the  cir  cum- 
stances  in  each  case.  Where  there  is  any  doubt  as  to  the 
position  of  the  crest  at  any  particular  depth,  to  make  sure  of 
reaching  the  oil  horizon  on  the  gently-dipping  flank  rather  than 
on  the  steep  flank  should  be  the  geologist's  endeavour.  Except 
in  long  anticlines  with  little  or  no  sign  of  dome  structure,  and 
where  the  oilpool  is  consequently  very  narrow,  there  should  be 
little  danger  of  missing  oil  if  the  effects  of  hade  are  carefully 
worked  out. 

When  flexures  are  intensely  folded,  or  even  overfolded,  as 
in  some  cases  in  Galicia  where  a  single  oil-bearing  stratum  has 
been  pierced  three  times  at  different  depths  in  the  same  well, 
the  conditions  are  apt  to  become  so  complicated  that  it  is 
impossible  to  state  any  general  proposition  that  will  serve  as 
a  guide  in  locating  wells  so  as  to  give  the  best  yield.  But  it 
is  as  well  to  remember  that  a  water-level  will  be  found  some- 
where in  almost  every  oil-bearing  rock,  however  well  isolated 
by  surrounding  impervious  beds.  The  geologist,  in  estimating 
the  area  from  which  a  production  of  petroleum  is  probable,  and 
the  area  likely  to  be  drained  by  a  well,  must  go  by  such  evidence 
as  is  available  either  from  other  wells  in  the  same  area  or  from 


140  OIL-FINDING 

productive  wells  in  other  areas,  assuming  that  an  oil- water-level 
will  be  discovered,  and  leaving  the  local  variations  in  this  hypo- 
thetical plane  between  water  and  oil  to  be  proved  by  actual 
drilling.  Local  variations  due  to  differences  in  porosity,  split- 
ting, thinning  out,  or  lenticularity  of  porous  beds,  and  seepage 
across  fault-planes  are  very  common,  but  cannot  be  reckoned 
upon  till  proved  by  the  evidence  from  a  number  of  bores. 

In  locating  wells  to  prove  the  extent  of  a  field  in  which  oil 
has  already  been  struck,  the  geologist  must  use  his  common 
sense  when  guiding  evidence  is  deficient.  Thus  if  an  anticline 
exhibits  dome  structure,  that  is  to  say,  if  well- defined  pitches 
point  to  the  length  of  the  field  not  being  excessive  in  com- 
parison with  its  breadth,  the  oil  reservoir  in  each  productive 
stratum  will  be  deep,  and  it  may  be  possible  to  locate  profitable 
wells  far  down  the  flanks  or  pitches  of  the  flexure,  while  if  the 
fold  be  little  affected  by  pitches  a  shallow,  long,  and  narrow  oil 
reservoir  may  be  expected.  In  any  case  the  geologist  will  find 
it  expedient  to  feel  the  way  cautiously  towards  the  limits  of 
an  oilpool,  rather  than  to  locate  wells  rashly  in  the  hope  of 
proving  a  wide  field  at  once.  It  is  hardly  ever  profitable  to 
drill  an  unsuccessful  well,  as  the  evidence  it  furnishes  is  almost 
entirely  negative,  and  does  not  necessarily  assist  those  in  charge 
of  drilling  operations  in  defining  the  limits  within  which  profit- 
able productions  can  be  obtained.  On  the  other  hand  when 
water  and  oil  are  found  in  the  same  stratum  when  pierced  by 
a  well,  when  the  occurrence  of  the  two  liquids  can  be  demon- 
strated in  intimate  association,  very  valuable  evidence  as  to  the 
extent  of  the  oilpool  may  be  furnished.  As  stated  above,  it 
is  useful  and  even  necessary  to  assume  that  there  is  a  regular 
level  between  oil  and  water  in  each  bed,  a  horizontal  plane 
above  which  oil,  and  below  which  water,  will  be  struck,  but  in 
actual  practice,  especially  where  the  oil  is  of  high  specific 
gravity,  it  may  be  exceedingly  difficult  to  determine  where 
such  a  plane  can  be  drawn  in  horizontal  sections.  In  simple 
and  well-defined  structures,  where  the  porosity  of  the  oilrocks 
is  fairly  constant  and  the  oil  of  light  gravity,  there  may  be 
little  difficulty,  but  even  in  such  a  case  the  plane  may  be  at 
different  levels  on  opposite  sides  of  an  anticline.  It  is  a  useful 
convention,  but  it  must  not  be  regarded  as  a  hard  and  fast  line 
which  cannot  be  affected  or  altered  by  local  conditions.  There 
are  many  cases  on  record  of  water  being  struck  in  a  well  and 


LOCATION    OF   WELLS  141 

pumped  for  months  before  oil  has  made  its  appearance  in  any 
appreciable  quantity,  and  yet  the  well  has  finally  yielded  oil 
without  any  admixture  of  water  and  continued  to  give  a  profit- 
able production  for  years.  Thus  the  actual  striking  of  water 
where  oil  is  expected  does  not  always  mean  that  the  well  is  a 
failure.  Again,  what  is  called  a  "freak  well" — a  deplorable 
phrase — may  be  brought  in  outside  what  has  previously  been 
accepted  as  the  limits  of  profitable  drilling.  Of  such  freaks 
there  is  always  an  explanation,  though  it  may  be  by  no  means 
obvious ;  in  many  cases  such  so-called  freaks  could  have  been 
foretold,  had  the  geological  conditions  been  studied  with  suffi- 
cient care. 

Many  of  the  discrepancies  between  predictions  and  results 
nowadays  are  attributed  to  lenticularity  of  the  oil-bearing 
strata.  Oilsands  are  doubtless  lenticular,  as  deltaic  and 
estuarine  deposits  must  necessarily  be,  and  as  for  that  matter 
every  clastic  deposit  in  the  world  must  be.  Among  the  rapidly 
deposited  sediments  of  a  delta  thinnings  out  and  variations  are 
naturally  especially  conspicuous,  but,  all  things  considered,  the 
lenticularity  of  oilsands  is  being  made  too  much  of.  To  shelter 
oneself  behind  "  that  comfortable  word "  lenticularity  when 
predictions  as  to  the  depth  and  position  of  oil-bearing  strata, 
or  the  prospects  of  a  well,  have  gone  astray  is  a  confession  of 
weakness,  ignorance,  or,  still  more  probably  the  want  of  careful 
detailed  mapping,  which  the  geologist  should  be  ashamed  to 
make  unless  he  is  in  a  position  to  prove  out  and  out  that  such 
lenticularity  exists.  As  a  general  rule,:if  he  can  prove  striking 
lenticularity  in  the  beds  exposed  at  the  surface  he  may  be 
justified  in  assuming  it  among  beds  of  similar  character  and 
mode  of  formation  underground.  In  any  case  he  should  be 
able  to  ascertain  the  general  directions  of  lateral  variations, 
and  should  thus  have  the  key  to  any  problem  involving  the 
sudden  thinning  out  of  beds  of  porous  strata  capable  of  con- 
taining petroleum,  or  the  sudden  appearance  of  such  strata. 
To  depend  upon  well-records  for  such  evidence  is  at  the  best  to 
obtain  information  at  second-hand,  and  it  is  not  in  every  field 
that  well-records  can  be  implicitly  relied  upon,  the  personal 
equation  entering  into  them  to  such  an  extent  that,  even  where 
carefully  kept,  they  may  leave  many  essential  points  doubtful. 
To  advocate  drilling  down  the  pitch  or  the  flank  of  a  flexure  in 
the  hope  of  striking  a  lenticular  bed  impregnated  with  oil  and 


142  OIL-FINDING 

sealed  from  the  invasion  of  the  dispossessing  fluid,  water,  by 
being  surrounded  by  impervious  strata,  is  to  reduce  geological 
science  to  the  level  of  guess-work.  Yet  wells  have  been  suc- 
cessfully brought  in  under  such  conditions  and  have  proved 
very  remunerative,  though  the  locations  have  been  disapproved 
of  by  geologists  on  grounds  perfectly  justifiable.  It  is  such 
instances  that  have  often  discredited  geological  work  in  the 
minds  of  practical  and  unscientific  oilmen,  and  it  makes  the 
geologist's  task  all  the  more  arduous  to  know  that  unexpected 
and  even  unprecedented  conditions  may  falsify  the  conclusions 
at  which  he  has  arrived  after  the  most  careful  consideration  of 
structural  and  practical  evidence  from  every  point  of  view. 
It  is  for  this  reason  that  the  study  of  lateral  variations  has 
been  insisted  upon  with  such  emphasis  ;  oilsands  can  be  shown 
to  be  splitting  up,  thinning  and  dying  out  by  evidence  visible 
at  the  surface,  as  the  directions  of  such  splitting,  thinning,  and 
dying  out  can  be  ascertained  beyond  question;  is  it  unjusti- 
fiable to  assume  that  similar  variations  must  exist  beneath  the 
surface,  and  that  from  what  can  be  actually  seen  we  may 
interpret  the  subterranean  anomalies  of  which  we  only  obtain 
direct  evidence  through  the  drilling  of  wells  ?  Lenticularity  of 
beds  may  be  a  very  important  factor  in  oilfield  work,  but  to 
assume  it  as  an  explanation  of  facts  that  have  not  been  antici- 
pated may  be  merely  a  begging  of  the  question.  In  locating 
wells  upon  an  anticline,  especially  if  it  be  of  considerable 
extent  and  length,  all  these  matters  must  be  considered,  and 
it  is  rash  to  assume  that  an  oil  horizon  proved  at  one  end  of 
an  anticline  must  necessarily  persist  to  the  other  end,  even 
where  the  structure  is  eminently  favourable  for  a  production  of 
petroleum. 

In  locating  wells  upon  a  monocline  or  a  terrace- structure, 
the  geologist  has,  as  a  rule,  a  very  simple  task.  He  will  be 
guided  first  of  all  by  any  local  variations  of  dip  or  strike  that 
may  be  observed,  and  secondly  by  the  presence  of  surface 
indications.  Where  the  dip  decreases  locally,  or  where  there 
is  a  sudden  change  of  strike,  especially  if  the  bend  in  the 
strike  is  concave  towards  the  direction  of  dip,  the  locality 
will  generally  have  better  prospects  of  production  than  areas 
lying  to  either  side.  In  a  terrace-structure,  where  the  oil- 
bearing  strata  do  not  crop  out  at  surface,  this  has  been  proved 
in  many  instances ;  in  monoclines  attention  is  usually  called  to 


LOCATION    OF  WELLS  143 

such  favourable  localities  by  the  "  shows  "  at  outcrop,  for  it  is 
at  such  changes  of  dip  or  strike  that  the  petroleum  tends  to  be 
concentrated  and  frequently  appears  at  the  surface. 

It  only  remains  to  calculate  at  what  depth  it  will  be  advis- 
able to  strike  the  oil-bearing  rocks,  to  measure  off  a  sufficient 
distance  in  the  direction  of  dip,  and  mark  the  location.  On 
terrace-structures  it  may  not  be  possible  to  calculate  the  depth, 
and  the  procedure  will  be  as  in  the  case  of  gentle  anticlines 
with  a  slight  degree  of  asymmetry. 

When  locating  on  a  monocline  it  may  be  taken  for  granted 
that  a  water-level  will  be  found  somewhere,  though  it  is 
possible  that  both  oil  and  water  may  be  encountered  together 
throughout  a  considerable  thickness  of  strata.  This  depends 
largely  upon  the  specific  gravity  of  the  oil,  and,  as  by  gradual 
inspissation  at  and  near  the  surface  the  oil  in  an  outcropping 
petroliferous  band  must,  however  slowly,  lose  its  lighter 
constituents  and  become  heavier,  a  final  stage  may  be  reached 
when  the  oil  approximates  in  specific  gravity  so  nearly  to 
that  of  water  that  replacement  by  the  latter  cannot  be  complete ; 
consequently  a  definite  water-level,  even  if  proved  in  one 
locality,  may  not  be  constant  over  any  considerable  distance 
in  the  outcropping  oilrock.  But  it  is  as  well  to  assume  that 
a  water-level  will  be  reached  sooner  or  later,  and,  therefore, 
the  oilrock  must  not  be  struck  at  too  great  a  depth.  At  too 
shallow  a  depth  gas-pressure  may  not  be  great  enough  to 
ensure  a  good  production,  and  the  oil  may  be  too  much  affected 
by  inspissation.  It  follows  that  the  making  of  a  location 
requires  the  exercise  of  judgment  and  will  be  governed  chiefly 
by  experience  of  results  obtained  in  similar  strata  and  structures 
and  with  similar  oils.  Localities  where  the  dip  is  lowest  will 
be  selected  in  preference  to  those  where  the  inclination  of  the 
strata  is  considerable  for  several  reasons;  in  the  first  place 
because  it  is  then  possible  to  place  the  well  further  from  out- 
crop for  a  given  depth,  and  secondly  because  seepages  at 
outcrop  may  not  have  depleted  the  petroliferous  bands  to  such 
an  extent.  A  depth  of  400  feet  is  very  suitable  for  a  first 
well  when  the  strata  are  inclined  at  an  angle  of  20  degrees  or 
less.  This  gives  a  minimum  distance  from  outcrop  of  between 
1100  and  1200  feet.  If  the  strata  dip  very  gently,  the  depth 
need  not  be  so  great  in  a  first  test.  After  one  successful  well 
has  been  drilled,  the  next  can  be  placed  to  strike  the  oilrock 


144  OIL-FINDING 

at  greater  depth,  and  the  limits  of  the  area  which  will  prove 
profitable  to  drill  felt  for  cautiously. 

With  beds  dipping  at  45  degrees  or  more,  600  feet  will 
not  be  too  great  a  depth  for  the  first  test- well.  In  the  case 
of  light  paraffin  oils,  as  has  been  explained  before,  such  tests 
may  be  quite  unsuccessful,  but  with  oils  of  asphaltic  base 
excellent  results  may  be  obtained  under  such  conditions. 

The  calculation  of  depth  is  a  matter  of  great  importance, 
especially  as  the  shutting  off  of  any  water-sands  that  may  be 
found  above  the  oilrocks  is  absolutely  essential  if  good  results 
are  to  be  obtained.  Given  a  careful  geological  survey  of  the 
area  there  should  be  no  difficulty  in  calculating  the  position 
of  the  oilrocks  and  the  water-sands  beneath  the  surface  at  any 
point,  and  it  may  be  possible  even  to  draw  contour  lines 
showing  the  approximate  depths.  But  the  field-student  must 
be  warned  against  projecting  the  angles  of  dip  as  observed  at 
the  surface  and  so  attempting  to  delineate  the  underground 
structure.  Such  methods  as  those  used  by  the  mining  engineer 
in  calculating  at  what  depth  a  shaft  must  be  sunk  in  any 
locality  to  strike  a  lode  will,  if  applied  to  oilfield  work,  often 

1  give  results  so  inaccurate  as  to  be  useless  for  practical  purposes. 
It  must  be  remembered  that  any  monocline  or  any  inclined 
bed  represents  part  of  the  great  curve  of  an  earthwave,  and 

I  that  the  part  seen  at  the  surface  is  infinitesimal  compared 
with  the  part  concealed  beneath,  so  that  the  angle  of  dip, 
however  carefully  measured,  may  not  be  very  useful  as  a 

!  guide.  The  drawing  of  horizontal  sections  to  scale,  when  there 
is  sufficient  evidence,  will  make  this  obvious  at  once,  and  will 

1  emphasize  the  futility  of  projecting  a  dip  as  seen  at  surface, 
as  if  it  continued  indefinitely  without  increase  or  decrease. 
It  is  expedient,  therefore,  to  make  a  careful  horizontal  section 
before  attempting  to  make  locations,  provided,  of  course,  that 
the  section  is  made  to  scale  from  a  geological  map,  and  is  not 
merely  the  diagrammatic  absurdity  produced  by  an  observer 
who  has  made  no  serious  attempt  to  map  the  ground  geologically. 
Thus  we  come  back  to  the  proposition  stated  above  that  the 
location  of  wells  should  depend  entirely  on  the  geological 
mapping,  and  provided  that  this  has  been  done  with  reasonable 
care  there  can  be  little  doubt  as  to  where  a  test-well  should 
be  placed. 

It  would  serve  no  useful  purpose  to  take  every  kind  of 


LOCATION    OF   WELl!S 


H5 


geological  structure,  and  give  in  detail  an  account  of  the 
conditions  which  should  determine  the  site  for  a  well :  in  spite 
of  elaborate  classifications  of  structure,  all  structures  known 
in  an  oilfield  can  be  considered  under  two  or  three  compre- 
hensive heads.  But  a  few  words  are  necessary  about  areas  where 
faults  are  a  conspicuous  feature.  Great  care  must  be  exercised 
in  locating  wells  in  faulted  areas,  not  only  because  the  fault 
plane  if  pierced  during  the  drilling  may  be  the  cause  of  great 
mechanical  difficulties,  making  the  keeping  of  the  bore  vertical 
and  the  sides  from  caving  by  no  means  an  easy  task,  but 
because  the  presence  of  faults  in  the  near  neighbourhood  may 


FIG.  13. — Diagram  showing  how  a  small  fault  may  enable  a  well  to  tap 
a  great  thickness  of  oilrock  locally.  Arrows  show  movement  of  oil 
and  gas. 

have  great  effects  upon  the  production  of  the  well.  The  theory 
that  faults  affect  a  field  adversely  by  allowing  migration  of  oil 
along  the  fault- plane  has  already  been  dealt  with  and  disposed 
of,  but  by  allowing  communication  between  separate  oilsands 
across  the  fault-plane  a  dislocation  of  the  strata  may  have 
remarkable  results  (Fig.  13).  The  field-work  of  several 
observers  has  proved  that  many  of  the  greatest  fountains  in 
the  Baku  field  lie  close  to  the  line  of  a  fault,  which  has  made 
possible  communication  between  separate  oilsands,  which  are 
both  thick  and  numerous,  so  that  a  well  on  or  near  the  line 
of  fault  is  able  to  derive  oil  from  many  horizons,  and  to  tap 

L 


146  OIL-FINDING 

them,  so  to  speak,  all  at  once.  A  somewhat  similar  ease  can 
be  cited  from  the  Yenangyoung  field  in  Burma,  where  Mr. 
B.  F.  N.  Macrorie,  of  the  Burmah  Oil  Company's  Geological 
Staff,  has  shown  how  small  faults  of  little  structural  importance 
have  assisted  in  raising  the  production  of  certain  wells  far 
above  the  average,  and  limiting  the  productiveness  and  life 
of  others. 

As   a  general   rule   it   may   be   taken   that   it   is    always 
preferable  to  drill  on  the  upthrow  side  of  a  fault  rather  than 
on  the  downthrow  side.     The  reasons  for  this  are  easily  under- 
stood when  it  is  remembered  that  any  fault  can  be  theoretically 
replaced  by  a  sharp  fold;  on  the  downthrow  side  the  throw 
of  the  fault  may  be  sufficient  to  bring  the  horizon  of  an  oil- 
bearing    band   below   water-level,  while   with   normal   faults, 
hading  to  the  downthrow  side,  the  well  may  encounter  the 
fault  plane  and  get  into  considerable  mechanical  difficulties. 
In  many  cases  what  is  seen  as  a  fault  at  the  surface  becomes 
a    sharp  fold   when   traced    downwards   where   the  elasticity 
of    the    beds     is    greater,    especially    when    thick    and    soft 
masses   of  argillaceous  rock   are  present.     Faulting  when  it 
occurs  in  a  series  where  by  far  the  greater  part  of  the  strata 
is  impervious,  and  the  porous  oilrocks  widely  separated,  may 
be  of  great  importance,  as  an  oil-bearing  band  that  would  other- 
wise be  found  cropping  out  at  the  surface  may  be  cut  off  and 
isolated  among  the  impervious  strata.     The  oil  contents  may 
be    preserved   thus   from  inspissation   and   great  productions 
may  be  obtained  from  a  band  isolated  in  this  manner.     It  will 
be  observed  that  the  throw  of  the  fault  may  not  be  a  matter 
of  importance  in  this  case  ;  either  an  upthrow  or  a  downthrow 
may  effect  the  isolation.     It  is  obvious  that  careful  geological 
work  is  necessary  before  it  is  possible  to  locate  wells  to  take 
advantage  of  structures    such  as  that  shown  in  Fig.  13,  but 
in  many  fields  unexpectedly  large  productions  have  been  struck 
by  the  drill  entering  a  band  of  oilrock  which  has  been  pre- 
served from  weathering  and  the  loss  of  light  oils  by  being 
cut  off  in  a  similar  manner. 

Faults,  generally  speaking,  unless  they  are  dislocations  of 
great  size  and  throw,  are  more  helpful  than  harmful  in  an 
oilfield,  for  the  simple  reason  that  in  most  productive  fields 
the  total  thickness  of  impervious  strata  is  in  excess  of  the 
total  thickness  of  porous  rocks.  Their  presence  may  complicate 


LOCATION    OF   WELLS  147 

the  geological  map  and  make  the  calculation  of  the  depth  to 
be  drilled  in  a  well  more  difficult,  but  their  presence  need  not 
have  any  deleterious  effect  upon  production. 

Questions   of    accessibility,    proximity    to    water    supply, 
expenses  of  road- making,  etc.,  must  all  be  taken  into  account 
when  making  a  location  for  a  test-well  in  a  new  field,  but  all 
these  matters,  though  serious  items  in  expenditure  accounts, 
must  be  regarded  as  secondary  to  finding  the  site  most  favour- 
able  according    to    the    geological    conditions.      The    young 
geologist  may  have  pressure  brought  to  bear  upon  him  to  fix 
upon  some  alternative  location  which  seems  "  almost  as  good  " 
as  the  one  he  had  originally  selected,  or  which  may  perhaps  be 
in  a  locality  where  the  prospects  of  obtaining  oil  are  doubtful, 
but   which   is    much   more    easily   accessible    and    will    not 
necessitate  any  great  expenditure  in  road-making,  transporta- 
tion of  plant,  and  furnishing  with  a  water  supply.     He  will  do 
well  to  resist  all  such  suggestions,  because  it  is  a  short-sighted 
policy  that  advocates  a  first  test-well  in  any  but  the  most  pro- 
mising locality  available.     The  cost  of  drilling  a  deep  test-well 
in  a  new  field  is  usually  so  greatly  in  excess  of  the  expenses 
incurred  in  road-making,  providing  water  supply,  etc.,  that  these 
may  be  disregarded.     If  the  more  accessible  site  be  chosen,  and 
after  months,  or,  if  any  difficulties  be  encountered  in  the  drilling, 
perhaps  more  than  a  year  spent  in  completing  a  deep  test  with- 
out successful  results,  another  well  costing  probably  nearly  as 
much  and  taking  as  long  to  drill  will  have  to  be  tried  before 
the  area  can  be  considered  fairly  tested.     On  the  other  hand  it' 
the  best  site,  geologically  speaking,  be  selected  at  first,  and  the 
test  be  unsuccessful,  the  area  may  be  abandoned  at  once,  and 
all  the  time  and  expense  of  drilling  a  second  well  saved.     It 
may  often  be  difficult  to  convince  field-managers  or  managing 
directors  that  an  area  can  be  thoroughly  tested  by  the  drilling 
of  one  well,  but  if  the  geological  work  has  been  done  thoroughly 
one  test  should  be  sufficient  in  almost  every  case,  and  when  the 
first  test  is  unsuccessful  the  throwing  away  of  time  and  money 
by  making  further  tests  is  a  matter  the  blame  of  which  must  be 
largely  at  the  door  of  the  geologist,  unless  his  advice  has  been 
arbitrarily  overruled. 

Very  frequently  a  geological  adviser  finds  himself  in  the 
position  of  having  to  advocate  the  testing  of  an  area  to  a  certain 
depth,  and  after  that  depth  has  been  reached  without  striking 


148  OIL-FINDING 

oil  it  may  be  necessary  to  say  at  once,  and  as  definitely  and 
strongly  as  possible,  that  there  is  no  further  hope,  and  that  the 
area  should  be  abandoned.  In  such  a  case,  if  the  well  be  "  in 
good  shape  "  to  be  carried  much  deeper,  there  may  be  consider- 
able hesitation  on  the  part  of  those  responsible  for  the  practical 
operations  in  deciding  to  abandon  it.  The  geologist,  having  the 
courage  of  his  own  convictions,  should  make  things  as  easy  for 
the  field-manager  as  he  can,  by  putting  the  case  clearly  and 
concisely  before  him.  Little  blame  can  be  attached  to  the 
unsuccessful  testing  of  a  new  field  by  drilling  one  well,  as  it  is 
often  impossible  to  make  sure  of  the  petroliferous  character  of 
part  of  a  series  in  any  particular  locality  without  evidence  from 
a  borehole ;  but  to  allow  a  second  unsuccessful  test  to  be 
drilled,  or  the  first  to  be  continued  to  a  great  depth  when  it  has 
no  further  prospect  of  striking  oil,  is  a  confession  on  the  part  of 
the  geologist  of  the  uncertainty  of  his  own  judgment  or  his 
ability  in  reading  the  evidence  obtained  during  the  geological 
mapping  of  the  ground.  Hence  it  becomes  of  the  utmost  im- 
portance that  no  testing  of  a  new  field  should  be  commenced 
till  the  geological  examination  has  been  made  in  detail  and  the 
location  made  in  the  best  possible  place  to  obtain  a  production 
of  oil.  Were  the  importance  of  this  principle  more  fully 
realized,  the  popular  idea  of  the  capricious  nature  of  petroleum 
would  be  shaken,  and  might  even  be  relegated  to  the  liinbo  of 
scientific  fallacies. 

After  a  successful  well  in  a  new  field  has  been  drilled,  the 
second  test  should  be  placed  so  as  to  develop  as  large  an  area  as 
possible  without  taking  the  risk  of  getting  beyond  the  margin 
of  the  oil-reservoir.  This  is  to  enable  some  idea  of  the  area 
available  for  drilling  to  be  obtained  at  once.  When  there  is 
great  doubt  as  to  the  extent  of  a  field,  the  best  policy  to  adopt 
in  developing  it  must  necessarily  be  uncertain,  but  with  a  fairly 
accurate  idea  of  the  minimum  size  of  a  new  field,  drilling  pro- 
grammes and  transportation  of  plant  can  be  taken  in  hand  in 
the  most  economical  and  adequate  manner.  The  only  exception 
to  this  is  when  the  petroleum  is  required,  and  can  be  handled, 
at  once ;  new  wells  may  then  be  started  near  the  first  test. 
This,  however,  is  a  state  of  things-  by  no  means  usual  in  new 
fields. 

Second  and  third  tests  should  not  be  located  directly  down 
the  dip  from  the  first  producing  well,  but  at  some  distance  to 


LOCATION    OF   WELLS  149 

the  side,  so  as  not  to  interfere  with  the  supply  of  oil  to  the  first 
well.  The  natural  migration  of  petroleum  will  be  up  the  dip- 
slopes  in  most  fields,  whether  in  monoclinal  or  anticlinal 
structures.  If  the  first  test- well  has  been  placed  on  one  flank 
of  a  symmetrical  anticline,  the  second  may  be  located  on  the 
other  flank,  in  order  to  obtain  information  as  to  the  breadth  of 
the  field.  In  a  dome  structure  the  second  test  should  be  made 
in  the  direction  of  the  larger  axis  of  the  dome. 

The  distance  at  which  wells  may  be  placed  from  one  another 
without  mutually  affecting  their  production  is  a  question  upon 
which  it  is  impossible  to  dogmatize,  as  it  depends  upon  so  many 
factors,  such  as  the  porosity  of  the  oilrocks,  the  grade  of  the  oil, 
and  the  gas-pressure,  which  may  be  different  for  any  different 
field.  It  may  be  taken  as  an  axiom  that  in  any  given  field 
there  <  is  a  certain  minimum  number  of  wells  which  will 
exploit  the  area  most  profitably  and  economically.  To  drilj. 
more  than  that  minimum  number  will  not  ensure  the  pro- 
duction of  more  oil  in  the  long  run,  but  less,  for  though  pro- 
duction may  be  more  rapid,  gas-pressure  will  be  dissipated 
more  quickly,  and  thus  the  motive  force  that  brings  the 
petroleum  into  the  well,  and  perhaps  up  to  the  surface,  will  be 
to  some  extent  wasted.  Fields  such  as  Spindle  Top,  in  Texas, 
and  Twingon,  in  Burma,  might  have  had  very  much  longer 
lives  and  produced  much  more  oil  with  a  fraction  of  the  expense, 
had  there  been  any  regulations  to  prevent  over-drilling. ' 

With  light  paraffin  oils,  high  gas-pressure  and  porous  sands, 
a  distance  of  one  hundred  yards  between  wells  will  probably  be 
found  a  convenient  and  sufficient  distance.  When  the  sands 
become  partly  exhausted  or  clogged  near  the  bottoms  of  the 
wells  by  the  deposit  of  solid  paraffin,  new  wells  may  often  be 
drilled  with  profit  between  the  old  producers.  In  shallow 
fields  with  asphaltic  oils,  and  in  oil-bearing  limestones,  wells 
may  be  placed  considerably  closer  without  seriously  affecting 
each  other,  but  in  each  field  the  requisite  minimum  distance 
must  be  ascertained  by  experience. 

In  calculating  the  number  of  producing  wells  which  a 
proved  area  will  carry  it  is  advisable  to  allow  for  a  distance  of 
from  200  to  300  feet  between  each. 

Though  it  is  no  part  of  the  geologist's  task  to  give  advice  as 
to  the  methods  to  be  made  use  of  in  drilling,  practical  experi- 
ence in  oilfields  will  soon  make  him  au  fait  with  the  chief 


150  OIL-FINDING 

mechanical  difficulties  that  the  driller  will  have  to  overcome, 
and  it  will  be  part  of  his  duties  to  acquaint  the  field-manager 
or  driller  with  the  nature  of  the  strata  through  which  the  well 
will  penetrate. 

This  will  enable  those  responsible  for  the  drilling  to  select 
the  best  methods  for  overcoming  the  difficulties  which  each 
kind  of  rock  will  present,  and  the  type  of  rig  and  tools  most 
suitable  will  be  chosen.  Thus  through  a  thick  soft  argillaceous 
group  it  may  be  found  most  profitable  to  use  a  rotary  rig,  while 
drop-drills  and  under-reamers  may  suit  a  variable  series  con- 
taining hard  calcareous  bands. 

The  approximate  depths  of  probable  water-sands,  the 
presence  of  hard  bands  upon  which  it  will  be  possible  to  ground 
casing,  the  occurrence  of  soft  beds  liable  to  cave  into  the  bore- 
hole, are  all  points  upon  which  the  geologist  may  give  infor- 
mation that  will  be  of  great  value  to  the  practical  driller.  Again, 
the  angle  of  dip,  if  it  is  high,  is  an  important  matter,  since 
steeply  inclined  beds  are  frequently  liable  to  cave,  and  if  thin 
hard  beds  are  encountered  dipping  at  a'high  angle  there  may  be 
great  difficulty  in  keeping  the  bore  vertical. 

Thus,  in  return  for  the  information  afforded  to  him  by  the 
log  of  a  well  the  geologist  should  be  able  to  forewarn  the  driller 
of  difficulties,  and  so  ensure  that  they  are  taken  in  hand  and 
overcome  most  expeditiously. 


CHAPTER    IX 

(FOR  BEGINNERS) 
FIELD    WORK 

IN  the  preceding  chapters  allusions  to  geological  mapping  have 
necessarily  been  very  frequent,  and  it  is  hardly  necessary  at 
this  stage  to  insist  that  the  object  of  all  geological  field-work 
must  be  in  the  end  to  make  as  complete  a  geological  map  as 
possible.  No  casual  examination  of  an  area  is  sufficient,  no 
spending  of  a  few  hours,  or  even  a  few  days,  if  the  area  be 
large,  in  examination  of  sections  and  oilshows  and  the  taking  of 
notes  will  qualify  the  geologist  or  petroleum  expert  adequately 
to  advise  those  who  are  undertaking  development  work.  It  used 
to  be  one  of  the  distinguishing  points  between  the  amateur  and 
the  professional  geologist  that  the  former  was  frequently  content 
with  the  drawing  of  a  horizontal  section,  while  the  latter 
alway  pinned  his  faith  to  a  map,  but  nowadays  the  amateur 
is  learning  that  in  any  case  the  map  must  be  made  before  the 
section,  and  that  nothing  but  a  map  will  suffice.  In  oilfield 
work  the  whole  concession  or  area,  and  frequently  a  large  area 
outside  of  it,  must  be  mapped  geologically. 

In  some  cases  a  published  geological  map  may  be  available, 
and  may  be  of  great  assistance,  but  it  is  not  likely  to  be  on  a 
sufficiently  large  scale  to  give  the  details  which  are  essential,  if 
wells  are  to  be  located  with  accuracy  to  strike  the  oil-bearing 
deposits  at  the  determined  depth.  The  best  topographical  map 
available  must  be  procured,  and  if  it  be  on  too  small  a  scale  it 
may  at  least  serve  to  check  distances  and  compass-bearings  in 
the  large  scale  map  which  the  geologist  will  prepare  for  himself. 
The  smallest  scale  that  is  at  the  same  time  sufficiently  large  to 
admit  of  mapping  in  detail  will  be  naturally  selected ;  for  most 
fields  and  field-geologists  the  scale  of  six  inches  to  the  mile  will 
be  found  to  meet  the  case.  Eight  inches  to  the  mile  is  also  a 
very  useful  scale,  and  in  producing  fields  scales  of  sixteen  or 


152  OIL-FINDING 

twenty-four  inches  to  the  mile  may  be  used  with  profit,  and 
may,  indeed,  be  necessary;  but  for  all  practical  purposes, 
especially  in  new  fields  and  in  wild  and  unopened  country,  the 
six-inch  scale  is  probably  the  best. 

Experienced  geologists  will  pardon  the  writer  for  giving 
some  account  of  the  methods  of  field-mapping  that  he  has 
found  most  effective  under  different  conditions,  in  the  hope  that 
some  of  them  may  prove  of  value  to  the  prospector  or  field- 
student,  for  whom  this  little  book  has  been  written. 

Many  of  the  details  to  which  much  attention  is  given  in 
large-scale  mapping  in  Britain  can  be  neglected,  partly  or 
wholly,  in  oilfield  work,  and  on  the  other  hand  methods  and 
conventions  that  are  not  required  in  ordinary  geological 
mapping  may  become  of  the  greatest  importance  when  an  oil- 
field is  being  surveyed.  It  is  necessary  that  structure  be 
worked  out  thoroughly,  and  it  is  for  this  reason  that  the 
mapping  must  be  done  on  a  scale  sufficiently  large  and  in 
sufficiently  great  detail  to  make  any  mistake  in  structure 
impossible.  But  the  nature  of  the  strata  and  the  mapping  of 
outcrops  with  great  accuracy,  and  determining  the  exact 
positions  of  points  may  in  many  cases  become  matters  of  minor 
importance.  The  determination  of  the  exact  position  of  the 
crests  of  sharp  anticlines,  the  angles  of  hade  of  the  axes  of 
asymmetrical  flexures,  and  the  pitches  of  axes  is  essential,  and 
consequently  observations  may  have  to  be  taken  very  frequently 
and  with  great  care,  while  the  angles  of  dip  on  the  flanks  of  a 
flexure  may  not  be  considered  of  sufficient  importance  to 
demand  any  special  care  in  the  taking  of  observations. 

Equipment. — The  geologist  who  undertakes  the  examination 
of  oilfields  must  have  an  effective,  but  not  necessarily  an  elab- 
orate equipment.  The  first  essential  is  a  good  and  substantial 
map-case.  The  large  leather  map-case  as  used  by  the  Geological 
Survey  of  Great  Britain  is  a  very  good  model,  though  it  may  be 
improved  in  details  to  suit  the  individual.  It  allows  six  square 
miles  of  ground  on  the  six-inch  scale  to  be  studied  at  one  time, 
without  changing  maps,  an  ample  area  for  all  practical  purposes. 
It  is  slung  from  the  shoulder  by  a  strap  and  can  easily  be 
manipulated  with  one  hand ;  this  may  seem  a  very  trivial  point, 
but  it  is  really  of  great  practical  importance.  Smaller  map- 
cases  or  mounted  and  folded  maps  carried  in  the  hand  or  in  a 
bag  or  pocket  will  be  found  troublesome  to  manipulate,  and  do 


FIELD   WORK  153 

not  conduce  to  good  geological  mapping.  The  tendency  will 
naturally  be  not  to  consult  the  map  frequently  enough,  and  the 
mapping  may  become  more  of  the  nature  of  taking  occasional 
notes.  The  possession  of  a  good  handy  map-case  opened  and 
managed  with  one  hand  will  do  much  to  teach  the  field-student 
practical  mapping  and  the  reading  of  geological  maps. 

Plane- tables,  though  excellent  for  careful  work  in  small 
areas,  are  too  cumbersome :  the  geologist  has  very  seldom  suffi- 
cient time  at  his  disposal  to  make  use  of  such  appliances,  and 
the  slight  gain  in  accuracy  obtained  by  using  them  is  more 
than  counterbalanced  by  the  laborious  nature  of  the  work  and 
the  waste  of  time  involved. 

Cavalry  sketching-boards,  fitted  with  a  compass  and  designed 
for  use  on  horseback,  are  pretty  little  toys.  They  may  be  of 
use  on  a  preliminary  traverse  or  a  pioneer  exploration  of  new 
countries,  but  they  are  too  small  for  detailed  and  accurate 
work,  while  the  compass  is  usually  also  too  small  to  take  bear- 
ings with  sufficient  accuracy.  Furthermore,  if  the  possession 
of  such  an  equipment  has  the  effect  of  inducing  the  young 
geologist  to  imagine  that  efficient  geological  work  can  be  done 
on  horseback,  it  may  be  his  ruin  so  far  as  practical  field-work 
is  concerned. 

For  instruments,  the  first  essential  is  a  good  pocket  compass, 
one  at  least  two  inches  in  diameter,  with  a  clearly  marked  dial, 
that  will  enable  the  observer  to  take  bearings  to  within  two 
degrees.  This  compass  should  be  carried  in  a  case  from  which 
it  can  be  taken  and  manipulated  with  one  hand.  The  saving 
of  time,  trouble,  and,  it  may  even  be  added,  temper,  that  is 
effected  by  carrying  a  compass  that  does  not  require  two  hands 
is  enormous  ;  this  can  be  understood  when  bearings  have  to  be 
taken  once  at  least  in  every  fifty  yards,  as  is  necessary  when 
working  in  dense  forest.  It  is  as  well  to  have  this  compass 
combined  with  a  clinometer  sufficiently  reliable  to  take  angles 
of  dip  without  an  error  of  more  than  one  or  two  degrees. 

For  taking  bearings  from  distant  points  a  good  large 
prismatic  compass  is  necessary ;  it  must  be  sufficiently  sensi- 
tive to  read  correctly  to  half  a  degree,  but  the  needle  must  not 
be  too  "  lively."  That  is  to  say,  though  sensitive,  the  card 
should  have  a  comparatively  high  "  moment  of  inertia."  This 
will  enable  readings  to  be  taken  by  the  method  of  oscillations, 
and  another  great  saving  of  time  will  be  effected.  The  geologist 


154  OIL-FINDING 

will  soon  learn  to  recognize  the  happy  mean  between  too  great 
mobility  and  too  great  sluggishness  in  a  prismatic  compass. 

An  Abne'y's  level,  or  some  similar  instrument,  is  sometimes 
necessary  in  taking  readings  of  the  angles  of  pitch  and  dip 
where  these  have  to  be  measured  very  carefully,  but  it  need 
not  be  carried  always.  In  producing  fields  and  open  ground  it 
is  far  more  likely  to  be  required  than  in  new  and  unexplored 
country. 

Theodolites,  tachyometers  or  omnimeters  are  often  of  great 
value  in  open  ground,  especially  where  there  is  no  topographical 
map  available,  but  it  is  impossible  for  the  geologist  to  carry 
such  instruments  with  him  in  rough  jungle  work.  The  young 
geologist  should  have  no  ambition  to  make  himself  a  third-rate 
land-surveyor,  and  though  it  is  necessary  to  understand  the 
use  of  these  instruments,  and  to  be  able,  if  it  is  required,  to 
measure  a  base-line  with  them,  he  will  be  well  advised  to  use 
them  as  little  as  possible ;  to  give  undue  attention  to  the  more 
or  less  mechanical  duties  of  land-surveying  may  run  away  with 
time  that  may  be  more  usefully  employed  in  geological  work. 
Like  the  Abney's  level  the  omnimeter  or  tachyometer  may  be 
left  at  headquarters,  and  only  taken  out  when  some  special 
work  with  it  becomes  necessary. 

A  good  protractor  adapted  to  the  scale  used  in  mapping 
must  be  procured.  This  may  have  to  be  made  specially  of 
ivory  or  aluminium  according  to  the  taste  of  the  geologist. 
Ivory  is  perhaps  the  better  material,  though  it  warps  badly  in 
hot  weather.  The  six-inch  protractor  used  by  the  Geological 
Survey  of  Great  Britain,  and  furnished  on  the  back  with  handy 
tables  to  enable  thicknesses  of  strata,  depths  and  gradients  to 
be  calculated  rapidly,  is  quite  the  best  instrument  of  the  kind 
for  six-inch  mapping. 

A  hammer  may  be  carried  if  required,  but  in  Tertiary  strata 
it  will  not  often  be  used;  a  cutlass,  machete,  dah  or  kukri 
will  be  as  effective,  and  will  serve  other  useful  purposes,  e.g.  in 
clearing  a  path  through  thick  jungle  or  in  digging  down  a  grass- 
grown  section  to  lay  bare  the  strata. 

A  stout  walking-stick  with  a  crooked  handle  by  means  of 
which  it  can  be  hung  on  the  arm  when  using  the  map-case  or 
compass  is  almost  invariably  carried  by  the  writer.  In  taking 
the  dip  of  a  ripple-marked  sandstone  it  may  be  laid  upon  the 
the  surface  of  the  rock  and  the  clinometer  placed  upon  it.  In 


FIELD   WORK  155 

slippery  or  soft  ground  or  in  rock-climbing  it  may  also  be  very 
useful,  and  in  tropical  countries  where  snakes  are  numerous  it 
may  be  necessary  as  a  weapon.  The  carrying  "of  a  stick  is, 
however,  a  matter  upon  which  the  individual  must  decide 
according  to  his  inclination. 

Pencils,  hard  or  soft,  will  be  chosen  to  suit  the  material 
upon  which  mapping  is  done,  and  the  climate,  whether  wet 
or  dry.  A  few  coloured  pencils  will  be  found  of  great  use,  and 
they  should  be  carried  so  that  the  colour  of  each  can  be  seen, 
and  any  one  selected  and  brought  into  use  with  one  hand.  A 
good  india-rubber  is  of  course  essential. 

As  to  the  material  on  which  the  mapping  is  to  be  done,  the 
author,  after  trying  many  varieties  from  tracing  linen  to  What- 
man's boards,  has  come  to  the  conclusion  that  oiled  paper 
mounted  on  linen  combines  the  greatest  number  of  advantages 
with  the  fewest  defects;  it  does  not  shrink  or  stretch  appreciably, 
it  is  not  rendered  useless  by  damp,  takes  pencil  and  chalk 
marks  clearly,  and  keeps  a  good  surface  even  after  much  rough 
usagp.  It  is  advisable  to  have  the  paper  cut  accurately  to  fit 
the  map-case.  Thus  for  the  ordinary  six-inch  map-case  the 
mapping-paper  should  be  cut  in  rectangles  of  twelve  by  nine 
inches. 

Some  observers  favour  squared  paper  for  field  work,  but 
if  it  is  really  to  be  of  use  it  must  be  adapted  to  the  scale  on 
which  the  mapping  is  done.  It  must  tend  also  to  make  field 
work  too  mechanical,  and  does  not  teach  the  field-student  to 
train  and  depend  upon  his  eye. 

A  note  book  is  often  useful,  but  is  not  absolutely  neces- 
sary ;  all  notes  of  importance  must  be  put  upon  the  field-map. 
Descriptive  notes,  lists  of  compass-bearings,  or  fossils  collected 
from  various  horizons,  and  small  details  of  mapping  or  sections 
shown  on  a  larger  scale  than  that  employed  on  the  map  can  be 
kept  in  note  books,  but  as  a  rule  all  these  can  be  put  in  con- 
densed form  on  the  field  map. 

Finally  a  strong  water-proof  bag  or  satchel,  capable  of 
holding  the  map-case  during  rainstorms,  and  with  an  extra 
pocket  for  other  instruments,  is  an  essential  part  of  the 
geologist's  equipment.  Willesden  canvas  is  a  very  suitable 
material  for  such  a  bag,  especially  when  bound  with  leather 
and  slung  on  a  strong  leather  strap  for  an  attendant  to 
carry. 


156  OIL-FINDING 

The  geologist  will  do  well  to  carry  all  the  instruments 
he  is  constantly  using  himself.  Hammer,  Abney's  level,  and 
occasionally  cutlass  and  prismatic  compass  may  be  carried  by 
one  of  his  attendants,  but  everything  else  should  be  disposed 
about  his  person  in  such  a  manner  that  it  can  be  brought  into 
use  with  the  least  delay  and  fumbling.  It  may  be  thought 
that  these  are  trivial  details,  the  neglect  of  which  can  be  of  no 
possible  consequence ;  but  if  the  field-student  has  to  work  in 
the  tropics  in  a  temperature  of  100  degrees  Fahr.  or  more  in  the 
shade  and  160°  or  170°  Fahr.  in  the  sun,  he  will  find  that  even 
trifling  details  become  of  importance,  and  trifling  annoyances 
may  be  magnified  into  trials.  To  have  to  wait  while  a  lazy 
native  servant  comes  up  with  the  instrument  required,  and 
slowly  unloads  a  bag  in  search  of  it,  to  have  to  hunt  for  a 
coloured  pencil  among  several  concealed  in  a  pocket,  when  the 
required  one  is  always  the  last  to  appear,  and  to  repeat  these 
performances  fifty  or  a  hundred  times  a  day  is  enough  to 
become  a  serious  worry  to  the  geologist  struggling  with 
climatic  conditions  to  which  he  is  not  accustomed,  and  his 
work  may  really  deteriorate  and  become  less  careful  through 
lack  of  attention  to  such  details.  Again,  the  time  occupied 
in  the  making  of  a  geological  survey  is  often  a  matter  of  great 
importance.  Kival  geologists  may  be  in  the  field,  other  interests 
may  be  represented  by  other  prospectors,  and  it  may  depend 
largely  upon  the  speed  with  which  the  main  points  of  a  struc- 
ture are  elucidated  that  the  success  or  failure  of  the  company 
or  syndicate  for  whom  the  geologist  is  acting  will  turn.  Every- 
thing, therefore,  that  favours  rapidity  in  field  work,  without 
decreasing  efficiency,  is  to  be  cultivated. 

Armed  with  the  equipment  set  forth  above,  the  geologist 
may  go  anywhere  and  map  any  ground  in  the  world,  provided, 
and  on  this  the  success  or  failure  of  his  work  depends,  that  he 
adapts  his  methods  of  survey  to  the  particular  variety  of  ground 
with  which  he  is  dealing.  The  dense  forests  of  Central  or 
South  America  cannot  be  attacked  in  the  same  manner  as  the 
barren  hills  and  plains  of  India  or  Persia. 

It  is  presumed  that  the  aspirant  to  become  a  petroleum- 
geologist  has  had  some  training  in  geological  mapping  on  a 
large  scale  before  he  is  called  upon  to  attempt  the  survey  of 
a  new  territory,  and  if  he  has  had  experience  of  mapping  in 
Britain  on  the  splendid  six-inch  maps  of  the  Ordnance  Survey, 


FIELD   WORK  157 

he  will  start  with  a  great  advantage  over  others  who  have  not 
been  so  fortunate.  The  areas  which  he  will  have  to  survey  in 
new  countries  where  the  oilfields  of  the  future  are  waiting  for 
development,  have  in  all  probability  never  been  mapped  topo- 
graphically, and  he  will  have  to  start  with  blank  paper  and 
construct  his  own  map.  In  such  cases  everything  will  depend 
upon  the  methods  by  which  the  survey  is  conducted. 

Survey  in  Open  Ground. — If  the  ground  be  open  and  largely 
bare  of  vegetation,  the  matter  is  fairly  simple.  A  base-line, 
or  still  better,  two  base-lines  meeting  at  an  angle,  must  be 
measured  and  marked  clearly  on  low  and,  if  possible,  level 
ground,  where  their  extremities  can  be  viewed  from  the  sur- 
rounding country.  Triangulation  by  prismatic  compass  from 
and  to  the  extremities  of  these  base-lines  will  give  a  sufficient 
number  of  points  to  form  a  skeleton  upon  which  to  construct 
a  geological  map.  Of  course  such  a  method  is  not,  and  cannot 
be,  entirely  accurate,  as  the  readings  of  compass  bearings  with 
a  hand  prismatic  compass  cannot  be  vouched  for  within  less 
than  30',  but  a  map  can  easily  be  constructed  by  means  of 
numerous  readings  and  check  readings  that  will  be  quite 
accurate  enough  to  ensure  that  no  error  in  geological  struc- 
ture is  possible.  Should  the  area  prove  eventually  to  be  a 
productive  field,  careful  land-surveying  will  have  to  be  under- 
taken sooner  or  later,  and  topographical  maps  accurate  in 
all  details  constructed,  but  that  is  not  a  matter  for  the 
geologist. 

The  length  of  the  original  base-lines  will  depend  upon 
the  size  of  the  area  to  be  mapped,  and  the  nature  of  the 
ground;  a  quarter  of  a  mile  will  usually  be  sufficient.  The 
distance  must  be  measured  carefully  by  chaining,  or,  if  such 
instruments  be  available,  by  means  of  a  tachyometer  or  omni- 
meter.  An  alluvial  plain,  if  the  area  contains  such,  is  naturally 
the  best  place  for  such  measurements.  The  angle  between 
two  base-lines  must  be  read  very  carefully  by  means  of  an 
omnimeter  or  by  prismatic  compass.  The  positions  of  pro- 
minent features,  hill-tops,  isolated  rocks,  or  trees,  conspicuous 
bends  in  the  courses  of  streams,  etc.,  in  the  immediate  neigh- 
bourhood are  determined  by  taking  bearings  from  the  extremities 
of  the  base-lines,  and  thus  a  series  of  points  is  obtained  from 
which  secondary  points  of  importance  can  be  fixed  upon  and 
marked  on  the  map.  As  many  check  readings  as  possible 


158  OIL-FINDING 

should  be  taken  in  determining  new  points,  and  where  dis- 
crepancies occur,  and  the  triangle  of  error  is  large,  the  readings 
which  are  most  nearly  at  right  angles  to  each  other  must  be 
taken  as  the  most  reliable.  The  top  of  the  paper  upon  which 
the  mapping  is  done  should  always  be  taken  as  true  north, 
and  the  magnetic  variation  allowed  for  in  plotting  the  results 
of  the  observations  made:  if  the  variation  be  to  the  eastward, 
it  is  added  to  the  readings  of  the  prismatic  compass,  and  if  to 
the  westward,  subtracted.  Many  square  miles  can  be  mapped 
by  this  method,  beginning  with  base-lines  of  not  more  than 
a  quarter  of  a  mile  in  length,  and  the  resulting  map  should  be 
sufficiently  accurate  to  make  the  working  out  of  geological 
structure,  and  the  location  of  wells  to  test  the  area  matters  of 
practical  certainty. 

Topographical  details,  except  in  the  case  of  important  cliff 
or  river  sections  which  must  be  mapped  carefully,  can  be 
sketched  in  as  the  geological  work  proceeds,  and  must  be 
regarded  as  of  secondary  importance  to  the  purely  geological 
mapping. 

Once  the  skeleton  of  the  map  is  prepared,  the  mapping  in 
fairly  open  ground  will  not  be  a  matter  of  difficulty,  as  there 
will  always  be  some  point  visible  from  which  bearings  can  be 
taken.  The  principal  section  across  the  general  strike  of  the 
strata,  preferably  a  cliff,  river,  or  road-section,  will  be  mapped 
in  detail  in  order  that  the  natural  subdivisions  into  which  the 
strata  range  themselves  may  be  ascertained,  and  prominent 
groups  of  beds  differentiated  and  selected  for  following  out 
through  the  area.  Upon  the  selection  of  such  groups  a  great 
deal  depends,  especially  where  variations  are  frequent  and  rapid  ; 
unless  such  main  divisions  of  the  geological  series  can  be 
determined,  the  construction  of  an  efficient  geological  map  is 
impossible.  To  cover  an  area  with  innumerable  observations 
of  dip  and  strike,  however  carefully  taken  and  noted  on  the 
map,  is  not  geological  mapping  in  any  sense  of  the  word,  and 
may  be  a  mere  waste  of  time  since  both  strike  and  dip  faults, 
unconformabilities  and  lateral  variations  may  never  be  detected 
by  such  an  amateur  method,  and  even  pitches  and  dome 
structures  may  not  be  recognized  if  the  ground  be  rough  and 
much  cut  up  by  valleys. 

Frequently  the  strata  group  themselves  naturally,  and  the 
geological  boundary  lines  to  be  followed  are  obvious,  but  in 


FIELD   WORK  159 

very  many  cases  the  geological  series  consists  of  rapid  alterna- 
tions of  two  or  three  types  of  strata  repeated  over  and  over 
again,  and  it  becomes  necessary  to  select  a  few  well-marked 
beds  neither  too  near  nor  too  far  from  each  other  and  to  map 
their  outcrops  as  far  as  possible.  It  may  be  necessary  to  map 
the  outcrops  of  many  beds  before  one  is  discovered  that  persists 
and  maintains  its  characteristics  over  a  sufficient  area;  a 
prominent  sandstone  or  limestone  bed  may  thin,  split  up,  and 
die  out,  and  it  may  be  necessary  to  cross  to  a  lower  or  higher 
horizon  and  carry  on  the  mapping  of  another  band,  which, 
though  not  so  conspicuous  where  first  observed,  extends  further 
and  remains  recognizable  over  a  greater  area  than  the  bed  first 
selected.  It  is  better  to  map  a  thick  bed  or  small  group  of 
beds  than  a  thin  bed,  on  account  of  the  rapid  changes  due  to 
lateral  variation. 

Where  dips  are  steep  it  is  not  necessary  to  map  separately 
horizons  near  to  each  other,  as  the  structure  will  be  made  clear 
by  the  tracing  of  horizons  from  500  to  1000  feet  apart;  but  in 
areas  where  the  strata  are  gently  inclined  and  outcrops  con- 
sequently become  complicated  and  irregular,  horizons  separated 
by  no  jnore  than  150  to  200  feet  should  be  mapped.  In  an 
area  with  low  dips  towards  the  centre,  and  steep  dips  towards 
the  margins,  thin  groups  will  be  mapped  in  the  central  part,  and 
the  groups  differentiated  may  be  thicker  and  thicker  in  the 
outermost  portions. 

It  is  not  sufficient  to  map  a  number  of  sections  across  the 
strike  and  join  up  the  outcrops  of  the  groups  as  observed,  unless 
the  ground  is  sufficiently  bare  to  allow  the  outcrops  to  be  seen  all 
the  way  between  each  dip  section.  The  selected  beds  or  groups 
must  be  followed  and  mapped  to  detect  any  faults,  changes  of 
dip  or  strike,  unconformabilities,  or  lateral  variations.  This 
method  is,  of  course,  somewhat  more  arduous,  and  takes  up 
more  time  than  sketching  outcrops  between  the  mapped  sections 
in  which  the  various  groups  have  been  identified,  but  it  gives 
absolutely  certain  and  indubitable  results  and  brings  out 
evidence  which  might  be  missed  by  making  use  of  any  less 
careful  method.  Coloured  pencils  will  be  found  most  useful 
in  distinguishing  the  horizons  followed  on  the  field  maps;  in 
the  finished  map  the  areas  between  mapped  horizons  form  the 
separate  groups,  which  will  be  differentiated  by  well-contrasted 
colours  to  bring  out  the  structure  so  that  it  can  be  understood 


160  OIL-FINDING 

at  a  glance.  It  is  then  of  very  little  moment  whether  or  no 
the  various  groups  are  of  the  same  types  of  sediment  or  not, 
so  long  as  they  are  separated  by  mapped  horizons  and  are 
distinctly  coloured. 

In  open  and  bare  ground  as  in  Egypt,  Persia,  Baluchistan, 
and  parts  of  India  and  Burma,  there  is  seldom  much  difficulty 
in  selecting  groups  for  mapping  and  differentiation,  but  when 
vegetation  is  thick  or  the  ground  obscure  the  geologist  may 
have  considerable  trouble  in  subdividing  the  part  of  the  series 
that  he  is  dealing  with  into  such  groups  as  will  by  their  out- 
crops bring  out  the  geological  structure  most  clearly ;  it  is  in 
easy  and  open  ground  that  the  experience  is  gained  that  will 
enable  the  field-student  to  deal  effectively  with  more  obscure 
areas. 

Eye  Training. — One  point  is  of  the  greatest  importance  to 
the  young  geologist  who  is  undertaking  the  survey  of  new 
territory.  He  must  train  his  eyes  and  learn  to  be  as  much  as 
possible  independent  of  his  instruments.  In  bare  and  open 
ground,  where  one's  position  can  always  be  ascertained 
accurately  by  taking  cross-bearings  upon  known  points,  the 
tendency  is  naturally  to  rely  upon  such  observations,  with  the 
result  that  when  one  is  suddenly  confronted  with  a  densely- 
forested  area,  one  may  despair  of  ever  making  an  accurate 
geological  map  of  it,  and  may  content  oneself  with  the  observa- 
tion of  a  few  dips  and  outcrops,  the  result  being  that  a  geologist 
of  better  training  has  eventually  to  go  over  the  ground 
independently  and  do  it  all  over  again. 

To  begin  with,  the  geologist  must  learn  the  scale  upon 
which  he  is  mapping,  that  is  to  say,  he  must  become  so  familiar 
with  it  that  he  can  judge  a  distance  as  seen  on  the  ground 
before  him  and  mark  that  distance  upon  his  map,  without 
pausing  to  consider  how  many  yards  or  feet  it  is.  To  pace  or 
chain  a  distance  and  then  measure  it  off  on  the  map  by  means 
of  a  protractor  is  no  doubt  often  very  useful,  but  there  is  no 
reason  why  the  geologist  should  not  train  his  eye  by  estimating 
the  distance  before  he  measures  it ;  to  be  able  to  map  any 
distance  up  to  three  hundred  yards  or  a  quarter  of  a  mile 
without  making  any  measurement  is  a  very  valuable  asset  to 
the  field  geologist.  It  is  doubtful  whether  any  one  is  so 
favoured  by  nature  as  to  have  a  special  gift  for  the  estimation 
of  distances,  but  the  faculty  can  easily  be  acquired  by  constant 


FIELD   WORK  161 

practice,  and  distances  up  to  half  a  mile  have  been  mapped  in 
the  author's  experience  with  errors  of  not  more  than  thirty 
feet.  It  is  better,  however,  not  to  attempt  to  map  distances 
of  more  than  a  quarter  of  a  mile  without  some  checking 
observations.  It  must  be  remembered  always  that  estimates  of 
distance  are  apt  to  vary  greatly  according  to  the  light.  The 
length  of  a  coast-section  with  the  tropical  sun  beating  upon  it 
is  liable  to  be  underestimated,  while  the  length  of  a  shaded 
road-section  overhung  by  trees  or  a  distance  in  jungly  ground 
may  easily  be  overestimated.  Consequently  the  field-student 
should  be  constantly  practising  the  transference  of  a  distance 
as  seen  to  his  map  under  every  condition  of  light  or  shade, 
afterwards  pacing  or  chaining  it  and  correcting  any  error  he 
has  made. 

The  next  point  in  the  training  of  the  eye  is  learning  to 
transfer  observed  angles  to  the  map  without  the  aid  of  a  pro* 
tractor,  and  with  a  very  small  margin  of  error,  so  that  when 
bearings  are  taken  with  the  pocket  compass  the  observed 
angle  can  be  sketched  at  once.  This  faculty  can  be  acquired 
very  quickly  with  a  little  practice;  angles  of  45  degrees, 
30  degrees,  and  60  degrees  are,  of  course,  very  easily  drawn, 
and  the  eye  soon  becomes  efficient  in  estimating  smaller,  greater, 
or  intermediate  angles  quickly.  The  error  should  not  be  more 
than  2  degrees,  and  provided  that  bearings  are  not  taken  from 
points  more  than  a  quarter  of  a  mile  distant,  the  map  will  not 
suffer  in  accuracy.  When  bearings  are  taken  by  prismatic 
compass  the  protractor  must  always  be  used  and  the  angle 
laid  off  as  carefully  as  possible,  but  in  all  ordinary  field  map- 
ping with  a  compass  by  means  of  which  the  observer  can  read 
a  bearing  within  2  degrees,  and  with  an  eye  practised  in  the 
estimation  of  angles  on  the  map  to  within  2  degrees,  mapping 
can  be  done  at  a  rapid  rate,  and  with  wonderful  accuracy,  pro- 
vided that  each  observation  only  includes  a  short  distance. 
For  practical  purposes  the  distance  should  never  exceed  a 
quarter  of  a  mile. 

Having  cultivated  the  faculty  of  estimating  angles  and 
distances  with  fair  accuracy,  the  geologist  will  be  able  to 
make  traverses  with  pocket  compass,  starting  from  a  known 
point,  and  if  possible  finishing  also  at  a  known  point.  Such 
work,  it  may  be  objected,  can  never  be  entirely  accurate,  but 
it  must  be  remembered  that  it  is  absolute  certainty  as  to  the 

M 


162 


OIL-FINDING 


geological  structure  that  is  to  be  aimed  at  rather  than  meticu- 
lous attention  to  details  of  topography.  A  traverse  by  means 
of  pocket  compass  of  a  mile  or  a  mile  and  a  half  in  length 
should  not  terminate  with  an  error  of  more  than  forty  or  fifty 
yards.  Bearings  should  be  taken  when  possible  by  prismatic 
compass  at  distances  of  not  more  than  half  a  mile;  this  will 
prevent  any  error  from  being  made.  If,  however,  no  check 
readings  are  possible  till  the  end  of  the  traverse,  there  will 
nearly  always  be  an  error  to  correct.  This  should  not  be  done 
at  once,  but  a  "  correction  mark "  put  upon  the  field-map 
(Fig.  14),  and  a  new  start  made  from  the  correct  position  as 
determined  by  compass  bearings.  Afterwards,  when  the  maps 


• ^  \ 


FIG.  14. — Sketch  mapping  in  the  field.  C.  Start  of  traverse ;  A.  Finish  of 
traverse ;  B.  Point  actually  reached.  =^  Correction  mark.  Traverse 
starts  again  from  B. 

are  being  inked  in,  which  should  be  done  every  day,  the  error 
can  be  corrected.  If  the  traverse  has  been  very  faulty,  it  will 
do  no  harm  to  make  a  second  traverse  starting  from  the  other 
end ;  it  is  better  to  learn  thoroughly  the  scale  on  which  one  is 
mapping  than  to  depend  entirely  on  one's  instruments.  It  is 
not  recommended  that  pocket  compass  traverses  should  be 
carried  to  a  distance  greater  than  three  miles,  and  the  beginner 
may  find  even  that  distance  too  long.  In  fairly  open  ground 
it  will  never  be  necessary  to  traverse  any  such  distance  with- 
out being  able  to  check  one's  position  by  taking  bearings  from 
some  point  fixed  by  triangulation,  but  in  forest  land  a  traverse 


FIELD    WORK  163 

without  check  of  three  miles  or  more  may  frequently  be  neces- 
sary. It  is  in  bare  or  comparatively  open  ground  that  the  field- 
student  must  teach  himself  to  map  with  that  accuracy  which 
will  be  his  only  support  when  he  has  to  deal  with  dense  jungle, 
where  no  check  readings  are  possible,  and  where  the  man  who 
depends  on  his  instruments  rather  than  on  his  eye  may  feel 
quite  unable  to  construct  a  geological  map. 

Another,  but  less  important,  faculty  that  should  be  culti- 
vated is  the  estimation  of  the  dip  of  strata  without  making  any 
observation  of  it  with  a  clinometer.  This  is  a  more  difficult 
matter  than  the  estimation  of  distance.  When  it  is  possible 
to  look  at  a  bed  along  the  strike  the  matter  is  simple,  and 
every  geologist  should  be  able  to  read  the  angle  within 
2  degrees,  but  it  is  often  impossible  to  get  such  a  view  of 
the  strata,  and  perspective  views  of  dip  are  very  deceptive. 
Constant  practice,  however,  will  enable  the  geologist  to  esti- 
mate dips  very  quickly  and  accurately,  but  it  is  not  a  method 
to  be  used  constantly  without  checks.  Whenever  it  is  possible 
to  take  with  a  clinometer  an  observation  of  dip  that  represents 
approximately  the  true  inclination  of  the  beds,  and  this  does 
not  happen  so  frequently  as  the  text-books  would  suggest, 
the  instrument  should  be  used,  but  at  the  same  time  the  eye 
may  be  trained  by  estimating  the  angle  before  observation  is 
taken. 

Where  dips  are  really  of  great  importance,  as  in  producing 
fields,  or  when  a  series  of  observations  has  to  be  made  to  enable 
a  horizontal  section  to  be  drawn  and  the  depth  at  any  point  to 
an  oil-bearing  horizon  calculated,  readings  with  an  Abney's  level 
or  some  similar  instrument  up  or  down  dip  slopes  is  by  far  the 
most  reliable  method.  Strange  as  it  may  seem  it  is  in  this 
simple  operation  of  observing  a  dip,  probably  the  first  thing 
in  field  work  that  the  budding  geologist  learns,  that  most 
mistakes  are  made,  and  mistakes  that  may  have  very  serious 
results.  The  tendency  is  always  (and  this  applies  to  the  ex- 
perienced geologist  as  well  as  to  the  beginner)  to  exaggerate 
the  angle  of  dip.  Where  bedding  planes  are  not  well-bared  or 
exposed,  it  is  almost  invariably  that  dipping  most  steeply  that 
is  selected  as  offering  the  best  surface,  and  unless  a  number  of 
observations  in  the  immediate  neighbourhood  be  taken  and 
iveraged,  the  general  inclination  of  the  strata  will  be  put  at 
two\high  a  figure  in  degrees. 


1 64  OIL-FINDING 

Again,  and  this  is  especially  true  of  Tertiary  deltaic  and 
littoral  deposits,  the  dip  of  bedding  planes  may  be  at  a  very 
different  angle  from  the  general  inclination  of  the  series.  Even 
where  no  false  bedding  can  be  detected,  the  strata  probably 
have  not  been  deposited  in  a  horizontal  position.  Theoretically 
in  fact,  deltaic  deposits  are  not  deposited  horizontally,  and 
amidst  the  rapidly  varying  and  quickly  accumulated  deposits  of 
a  Tertiary  delta,  where  much  of  the  petroleum-geologist's  work 
will  be  done,  it  is  by  no  means  easy  to  make  sure  of  the 
average  inclination.  Where  folding  is  well  marked,  dips  may 
change  every  few  yards,  and  not  by  regular  gradations,  but 
often  suddenly,  so  that  quite  apart  from  irregularities  of  original 
bedding  the  determination  of  the  true  dip  at  any  point  may  be 
a  very  difficult  matter.  The  only  method  in  such  cases  is  to 
make  many  observations  on  all  sides,  if  there  be  sufficient 
evidence,  and  to  take  an  average  both  as  regards  strike  and 
dip,  always  remembering  that  the  minimum  inclination  ob- 
served is  more  likely  to  be  correct  than  the  inaxinmra.  Strike 
is  in  any  case  more  important  than  dip,  and  it  is  always  as  well 
to  mark  the  strike  of  a  bed,  even  when  it  is  impossible  to 
ascertain  its  true  dip.  It  is  because  dips  have  to  be  averaged, 
and  because  it  is  inadvisable  to  give  too  much  weight  to  a  few 
isolated  observations  of  the  inclination  of  strata,  that  the 
method  of  estimating  dips  by  the  eye  alone  is  frequently  suffi- 
cient for  all  practical  purposes. 

In  recording  on  the  map  an  observation  of  dip,  the  point  of 
the  arrow  should  be  marked  as  nearly  as  possible  on  the  spot 
where  the  observation  was  taken. 

Surveying  in  Jungles  or  Forest  Land. — Lack  of  evidence  is 
always  the  greatest  difficulty  in  the  way  of  making  an  intelligible 
and  accurate  map,  and  it  is  in  the  making  of  an  intelligible  and 
accurate  map  where  evidence  is  meagre,  that  the  experienced 
geological  surveyor  proves  his  ability.  Any  one  can  map  strata 
that  he  can  see  exposed,  but  where  exposures  are  few,  or  perhaps 
entirely  wanting  over  miles  of  country,  new  methods  have  to 
be  devised,  new  kinds  of  evidence  have  to  be  studied,  and 
nothing  may  be  too  small  and  insignificant  to  give  some  hint  as 
to  the  strike  or  dip  of  concealed  strata.  Unless  evidence  be 
studied  minutely  in  more  or  less  open  ground — such  matters, 
for  instance,  as  the  colour  and  texture  of  soils,  the  vegetation 
that  grows  on  different  varieties  of  deposit,  clay,  sandstone, 


FIELD   WORK  165 

limestone,  as  the  case  may  be,  outcrops  of  water  or  petroleum — 
the  key  to  the  structure  of  obscure  or  wooded  country  may 
be  lost. 

One  frequently  sees  it  stated  that  there  are  "  indications  of 
petroleum  "  in  a  certain  district,  but  that  "  it  is  impossible  to 
ascertain  the  geological  structure,  as  the  ground  is  too  densely 
clothed  with  vegetation."  In  other  words,  the  geologist  has 
been  unable  either  from  want  of  time,  want  of  sufficient  care, 
or  the  lack  of  reliable  methods  of  surveying,  to  determine  the 
geological  structure.  With  the  single  exception  of  alluviaj. 
flats  so  vast  in  extent  that  the  particular  area,  the  geological 
structure  of  which  is  in  question,  is  too  far  from  any  of  the 
margins  where  reliable  evidence  can  be  obtained,  there  is  no 
part  of  the  world's  land  surface  where  such  an  impossibility 
exists.  An  ice-sheet  may  be  considered  as  an  exception  to 
this,  but  it  is  hardly  to  be  regarded  as  a  land-surface. 

Alluvium  acts  as  a  sponge,  wiping  out  all  direct  evidence, 
though  where  belts  of  alluvium  are  not  very  large,  their  very 
presence  may  furnish  valuable  negative  evidence ;  but  no  other 
covering,  whether  of  glacial  drift,  blown  sand,  peat,  vegetation, 
or  coral  terrace,  is  sufficient  to  prevent  some  details  of  geological 
structure  being  found  somewhere.  It  is  with  the  dense  vegeta- 
tion difficulty  that  the  petroleum-geologist  has  to  deal  in  many 
parts  of  the  world.  Tropical  forests,  such  as  those  of  South 
and  Central  America,  or  the  bamboo  jungles  of  India,  are 
perhaps  the  most  disheartening  areas  in  which  to  attempt 
geological  mapping,  but  it  can  be  done  ;  geological  structure  can 
be  elucidated,  and  maps,  not  in  great  detail  or  of  great  accuracy, 
but  at  least  reliable,  can  be  made  even  under  such  conditions. 
The  secret,  if  secret  it  can  be  called,  is  simply  the  adapting  of 
one's  methods  to  the  particular  work  that  is  in  hand.  A  com- 
pletely accurate  map  is  perhaps  an  impossibility  without  great 
expenditure  of  time  and  money  in  trace-cutting  and  land  sur- 
veying, for  which  the  geologist  may  not  be  able  to  spare  the 
time,  nor  in  all  probability  will  he  have  the  necessary  instru- 
ments, but  a  sketch  map  of  sufficient  accuracy  can  be  pieced 
together  by  careful,  if  at  times  laborious,  work,  just  as  a  sketch 
map  may  be  made  anywhere  without  triangulation.  It  is  here 
that  the  observer,  who  has  thoroughly  mastered  his  scale  and 
can  map  accurately  on  pocket  compass  traverses,  has  the  advan- 
tage over  those  who  are,  so  to  speak,  tied  to  their  instruments. 


166  OIL-FINDING 

If  there  be  any  road  or  coast- section  crossing  or  skirting  the 
area  to  be  surveyed  it  must  be  examined  and  mapped  in  detail 
first,  copious  notes  being  taken  of  the  characteristics  of  each 
bed,  such  as  the  presence  of  pebbles  or  nodules  and  their 
natures.  In  a  road,  even  where  there  is  no  section  in  side 
cuttings,  it  is  possible  to  glean  a  fair  amount  of  information. 
For  instance,  those  parts  underlaid  by  clay  can  always  be  dis- 
tinguished from  parts  where  the  underlying  beds  are  arenaceous, 
and  a  sharp  and  distinct  line  between  thick  masses  of  arenaceous 
and  argillaceous  sediments  can  often  be  drawn  where  no  actual 
exposure  is  seen. 

Then  the  forest  or  jungle  must  be  attacked  as  far  as 
possible  in  the  same  manner  as  in  the  case  of  more  open  ground. 
It  is  presumed  that  there  is  no  topographical  map  available, 
that  no  hills  from  the  summit  of  which  compass  bearings  can 
be  taken  are  to  be  seen,  and  that  the  courses  of  such  streams 
and  rivers  as  flow  through  the  area  are  unknown.  A  coast, 
road,  or  river-section  may  give  the  key  to  the  structure  at  once, 
but  should  no  such  section  be  available,  or  should  it  be  discon- 
tinuous or  obscure,  it  can  only  serve  as  a  base-line  on  which 
starting  points  for  traverses  may  be  selected. 

To  begin  with,  if  any  group  of  hard  or  massive  beds  be 
present,  the  geologist  should  endeavour  to  follow  it  along  the 
strike,  noting  the  types  of  vegetation  it  supports,  the  colour  and 
texture  of  the  soil  it  forms,  and  whether  under  the  weathering 
processes  peculiar  to  forest  land  it  is  capable  of  standing  out  as 
a  marked  feature.  In  all  thickly  forested  country  there  must 
be  a  fairly  heavy  rainfall,  and  consequently  denudation  of  the 
surface  will  be  fairly  rapid  in  spite  of  the  protection  afforded 
to  the  soil  by  the  vegetation.  An  arenaceous  group  in  these 
circumstances,  however  soft  and  loosely  compacted  the  strata 
may  be,  will  always  tend  to  form  hills  and  high  ground  as  con- 
trasted with  argillaceous  strata.  Much  of  the  rainfall  is 
absorbed  by  the  porous  arenaceous  rocks  to  be  thrown  out  as 
springs  at  the  foot  of  dip-slope  or  escarpment,  whereas  an 
argillaceous  outcrop  absorbs  little  of  the  rainfall,  but  causes  it 
to  flow  over  the  surface,  thus  favouring  sub-aerial  denudation. 
Consequently  the  outcrop  of  an  argillaceous  group  among 
arenaceous  rocks  will  almost  invariably  be  marked  by  a  valley 
or  belt  of  low  ground,  however  tough  and  hard  the  material 
may  be,  and  an  arenaceous  group  among  clays  will  stand  out  as 


FIELD   WORK  167 

a  ridge,  however  loosely  compacted  the  strata  of  which  it  is 
composed.  In  bare  and  open  ground,  where  the  rainfall  is  not 
heavy,  the  relative  porosities  of  the  strata  do  not  have  such  a 
marked  effect  upon  the  contours  of  the  surface. 

The  mapping  of  surface  features,  therefore,  often  becomes 
very  important  and  of  the  greatest  help  to  the  geologist,  though 
it  must  not  be  relied  upon  unless  confirmed  by  other  evidence 
such  as  the  nature  of  the  soil.  Where  denudation  is  rapid  it 
may  produce  a  complex  system  of  ridges  and  valleys  that  have 
little  or  no  relation  to  the  strike  and  dip  of  the  strata ;  in  a 
thick  series  of  clays  in  which  the  physical  characters  of  different 
bands  differ  very  slightly,  an  irregular  and  complicated  drainage 
system  quite  irrespective  of  geological  structure  may  be 
established,  and  the  contours  of  the  country  where  they  can  be 
observed,  e.g.  in  areas  planted  with  sugar-cane,  may  be  sufficient 
to  show  that  the  strata  are  argillaceous  before  the  soil  has  even 
been  examined. 

The  angle  of  dip  has  also  to  be  considered  when  features 
are  being  mapped ;  the  greater  the  angle,  the  more  clearly 
marked  will  be  strike  features,  and  where  the  strata  are 
practically  horizontal,  outcrops  naturally  become  very  irregular 
and  the  following  of  them  in  undulating  forest  land  may  be 
simply  a  waste  of  time. 

Having  selected  a  group  of  strata  that  seems  likely  to  form 
good  strike  features,  and  that  is  dipping  at  a  sufficiently  high 
angle  where  it  is  observed  in  the  base-line  section  on  coast-line, 
road,  or  river,  it  should  be  followed  as  far  as  possible  along  the 
strike.  Exposures  may  be  few  or  entirely  wanting,  but  by 
studying  the  soil  and  the  vegetation  it  may  be  possible  to  follow 
a  group  for  great  distances.  The  occurrence  here  and  there  of 
loose  fragments  of  a  hard  rock,  e.g.  a  calcareous  sandstone, 
along  an  ill-defined  ridge,  may  enable  an  outcrop  to  be  picked 
up  and  'mapped  for  miles  till  a  river  valley  cutting  across  the 
strike  gives  an  exposure  and  allows  an  observation  of  dip  to  be 
made.  Once  an  horizon  has  been  traced  through  the  area  to  be 
examined  the  following  of  other  horizons  becomes  a  much  easier 
task,  and  a  fairly  complete  geological  map  may  be  constructed 
from  evidence  which  approached  by  any  other  method  would 
throw  very  little  light  upon  the  geological  structure. 

As  a  rule  it  is  better  not  to  follow  the  courses  of  streams  at 
first,  at  least  not  until  their  general  directions  are  asceitaiced, 


1 68  OIL-FINDING 

If  their  courses  be  tortuous  the  mapping  will  be  very  tedious, 
and  perhaps  will  result  in  the  discovery  of  little  evidence, 
while  alluvial  flats  may  be  encountered  to  the  discouragement 
of  the  observer.  Where  steep  dips  give  evidence  of  flexuring 
on  a  considerable  scale,  however,  the  courses  of  streams  or 
rivers  can  usually  be  resolved  into  "  consequent  "  portions, 
1  i.e.  across  the  strike,  and  "  subsequent "  portions,  i.e.  along  the 
strike ;  and  even  where  no  exposures  are  to  be  seen,  the  evidence 
from  the  directions  of  drainage  taken  in  connection  with  the 
orientation  of  ridges  and  hollows  may  give  valuable  evidence  as 
to  the  strike  of  the  series. 

In  following  up  outcrops  or  traversing  across  the  strike,  the 
geologist  must  map  by  "dead  reckoning"  with  his  pocket 
compass,  using  his  map  case  every  fifty  yards  or  so  to  mark  his 
track.  Where  the  jungle  is  thick  and  has  to  be  cutlassed  to 
allow  passage,  if  two  men  be  kept  in  front  of  the  observer  at 
intervals  of  from  10  to  20  yards  the  mapping  of  track  can  be 
simplified  by  omitting  many  of  the  minor  turns  and  twists 
inevitable  when  marching  in  forest  land.  It  is  not  recom- 
mended that  traverses  of  more  than  one  mile  be  made  at  first, 
while  three  miles  is  as  far  as  anyone  is  likely  to  be  able  to 
traverse  by  dead  reckoning  with  any  degree  of  accuracy ;  the 
writer  has  found  that  a  traverse  by  pocket  compass  of  four  or 
five  miles  in  forest  land  is  inadvisable  unless  it  is  to  a  known 
point,  or  to  a  point  the  position  of  which  can  be  ascertained  by 
taking  compass  bearings. 

The  time  required  for  simple  mapping  of  the  route  taken, 
without  study  of  geological  data,  will  vary  greatly  according 
to  the  nature  of  the  ground.  Where  there  is  not  much  cut- 
lassing to  be  done  and  slopes  are  not  too  precipitous,  one  mile 
an  hour  is  a  fair  average  pace.  In  difficult  country  and  where 
many  observations  have  to  be  made  the  pace  may  be  much 
slower. 

Checking  a  traverse  can  only  be  done  by  making  it  a 
"closed  traverse,"  coming  out  to  some  point  along  a  road, 
river,  or  coast-line  where  the  position  can  be  found,  or  by 
making  another  traverse  from  a  different  starting  point  to  the 
same  final  point. 

In  any  case  where  the  geologist  fails  to  keep  his  track 
mapped  and  does  not  know  his  position,  he  should  map  on 
a  new  sheet  of  paper  or  in  a  note  book,  and  either  begin  a 


FIELD   WORK  169 

fresh  traverse  from  his  unknown  position  to  reach  some  point 
which  he  can  fix  or  recognize,  or  take  a  compass  direction  and 
keep  it  as  straight  as  he  can  out  to  road,  river,  or  coast-line. 
It  is  always  better,  however,  to  follow  an  outcrop,  if  one  can 
be  recognized  and  followed,  than  to  map  track  across  bedding. 

It  may  seem  that  these  methods  are  very  rough  and 
uncertain,  and  there  is  no  doubt  that  the  geologist  when  he 
first  undertakes  work  in  tropical  forest  will  make  many  faulty 
traverses  before  he  becomes  master  of  the  scale  on  which  he 
is  working  and  capable  of  traversing  forest  up  and  down  hill, 
in  and  out  of  creeks  and  gullies  while  keeping  his  dead 
reckoning  with  accuracy,  but  there  is  no  other  method  that 
will  yield  results  so  quickly,  and  at  the  same  time  develop 
confidence  in  the  observer.  To  map  with  theodolite  or  plane 
table  in  the  forest,  cutting  traces  and  chaining  distances  is  far 
too  cumbrous  and  slow  a  method,  and  can  only  be  justified 
when  the  area  to  be  examined  is  very  small  or  when  the  exact 
position  for  a  test  well  is  being  determined. 

In  jungle  work  where  evidence  is  very  scanty  the  geologist 
must  be  continually  on  the  alert :  nothing  is  too  insignificant 
to  be  noted.  Every  change  in  the  colour  of  the  soil,  every 
ridge  that  does  not  run  parallel  to  the  drainage  channels,  every 
occurrence  of  loose  pebbles  or  nodules  should  be  noted  and 
the  note  marked  clearly  on  the  map.  Similarly  changes  in 
the  nature  of  the  vegetation,  if  they  are  sudden,  should  be 
mapped.  An  exposed  section  may  make  clear  the  reason  for 
such  a  change,  and  a  very  valuable  piece  of  evidence  may  be 
added  to  the  geologist's  store  of  accumulated  data.  In  Trinidad 
the  Cretaceous  formation,  which  lies  unconformably  beneath 
the  petroliferous  Tertiary  Series,  has  frequently  been  recognized 
by  the  colour  of  the  soil  and  the  nature  of  the  vegetation, 
when  no  exposures  of  the  strata  were  to  be  seen.  When 
exposed  the  strata  are  often  very  similar  to  some  of  the 
Tertiary  deposits,  but  the  soil  formed  by  the  disintegration 
possesses  some  pecularities  which  distinguish  it  from  that 
formed  from  any  of  the  Tertiary  strata.  Much  of  the  Cretaceous 
formation  has  been  prospected  for  petroleum  by  observers  who 
have  not  learnt  to  distinguish  it  from  the  overlying  Tertiary 
rock. 

In  clay  ground  the  different  tints  induced  by  weathering 
processes  have  often  proved  of  the  greatest  value,  and  have 


i/o  OIL-FINDING 

enabled  different  bands  to  be  mapped  witb  accuracy.  The 
black  soils  of  a  marl  outcrop  contrast  so  strikingly  with  the 
red  or  yellow  soils  derived  from  a  clay  that  there  need  be  no 
hesitation  in  mapping  them  separately.  Again,  "  outcrops  of 
water,"  surface  springs,  or  damp  ground  marked  by  the 
occurrence  of  water-loving  plants  and  trees  often  enable  the 
observer  to  draw  a  boundary  line  which  will  be  found  later 
to  coincide  with  the  outcrop  of  a  porous  stratum. 

Excavations. — The  making  of  excavations  to  ascertain  the 
nature,  dip  and  strike  of  strata  is  sometimes,  but  very  rarely, 
necessary.  False  evidence  obtained  by  this  method  has  often 
to  the  writer's  knowledge  led  observers  to  make  very  serious 
and  sometimes  even  ludicrous  mistakes  in  their  interpretation 
of  geological  structure.  It  must  be  remembered  that  in  forest 
land,  especially  in  tropical  countries,  disintegration  of  the 
strata  extends  for  a  great  distance  from  the  surface,  often 
upwards  of  thirty  feet,  and  in  hilly  ground  surface-slip  in 
partially  disintegrated  rock  causes  an  astonishing  amount  of 
modification  in  the  position  of  bedding  planes.  Root  growth 
also  disturbs  the  strata  for  a  considerable  distance.  The  result 
is  that  it  is  very  difficult  to  select  a  spot  for  digging  a  trench 
where  really  reliable  evidence  will  be  obtained  without 
excavating  to  a  great  depth.  Small  pits  and  trenches  are 
liable  to  be  dug  into  displaced  or  disintegrated  beds,  and  it 
will  readily  be  understood  what  confusion  may  arise  through 
accepting  the  false  evidence  obtained  by  this  method.  It  is 
only  natural  that  the  observer,  having  been  at  the  expense 
and  trouble  of  having  a  few  excavations  made,  should  attach 
more  importance  to  the  evidence  obtained  from  them  than  to 
the  possibly  more  obscure,  but  certainly  more  reliable,  evidence 
that  he  has  obtained  by  mapping  outcrops  or  by  the  examination 
of  natural  exposures.  And  thus  he  may  acquire  an  entirely 
incorrect  idea  of  the  geological  structure. 

There  is  something  to  be  said  for  the  digging  of  a  few 
pits  or  trenches  when  it  is  done  in  connexion  with  the 
mapping  of  outcrops,  but  to  depend  on  excavation  alone  to 
obtain  evidence  is  to  court  disaster.  In  mapping  some  500 
square  miles  in  the  island  of  Trinidad  the  author  only  made 
use  of  specially  dug  trenches  some  half  dozen  times,  and  then 
it  was  to  settle  some  detail  rather  than  for  general  purposes 
of  mapping.  Some  cuttings  on  roads  in  that  Colony  are 


FIELD    WORK 


171 


sufficient  to  prove  what  startling  changes  in  strike  and  dip, 
and  even  inversions,  in  the  soft  Tertiary  strata  are  due  to 
surface  slip. 

If  it  becomes  necessary  to  make  an  excavation,  it  is  impor- 
tant to  select  a  spot  where  evidence  should  be  obtained  without 
digging  deep,  and  where  such  evidence  is  likely  to  prove 
reliable.  The  bottoms  of  valleys  are  naturally  to  be  avoided, 
and  also  the  tops  of  hills,  hillocks,  or  plateaux ;  in  the  first 
case  there  will  probably  be  a  great  accumulation  of  surface 
wash  (Fig.  15),  while  on  the  tops  of  hills  there  may  be  a  great 


FIG.  15. — Excavation  for  dip  evidence.     1.  Disintegrated  strata  or 
surface  wash. 

thickness  of  completely  disintegrated  rock.      At  the  top  of  a 
sharp  ridge  or  hillock,  or  just  beneath  its  summit  (Fig.  16), 


FIG.  16. — Surface  curvature,  giving  false  dips  at  top  of  ridge. 

surface  curvature  may  vitiate  the  accuracy  of  the  observation 
although  the  strata  may  be  obviously  in  situ,  and  at  the  bottom 
of  such  a  ridge  water  may  collect  so  rapidly  as  to  hinder  the 
digging.  Half-way  down  a  steep  slope,  especially  if  the  slope 
is  at  a  high  angle  to  the  probable  line  of  strike,  gives  the  best 


i/2  OIL-FINDING 

chance  of  a  reliable  exposure,  while  the  work  of  making  the 
excavation  will  be  easier,  and  the  trench  or  pit  can  be  kept 
drained  and  the  exposed  rock  allowed  to  weather  if  the  bedding 
is  not  apparent  at  once.  In  many  varieties  of  Tertiary  strata 
it  is  easier  to  detect  the  bedding  planes  after  a  certain  amount 
of  weathering  has  taken  place,  so  that  the  keeping  of  an  exca- 
vation free  from  water  is  a  distinct  advantage.  But  even  with 
such  a  favourable  spot  selected,  false  evidence  may  be  obtained 
if  the  strata  be  largely  argillaceous.  It  is  among  the  alterna- 
tions of  arenaceous  and  argillaceous  beds,  and  where  bands  of 
hard  rock  or  nodular  concretionary  bands  are  present  that  the 
best  results  are  obtained  from  excavations. 

Where  flexuring  has  been  intense,  small  minor  folds  or 
wrinkles  may  be  occasionally  present  in  a  monocline  far  from 
any  important  anticlinal  bend.  This  may  lead  to  an  incorrect 
reading  of  the  geological  structure  if  the  observer  relies  upon 
excavations  for  his  evidence.  Fig.  17  shows  a  case  that  has 


FIG.  17. — Obscure  ground  local  flexure  giving  a  false  idea  of  general 
structure. 

actually  come  under  the  writer's  observation.  The  ground  was 
low  lying  and  evidence  was  very  scanty ;  only  at  the  points 
A  and  B  could  evidence  of  dip  be  obtained.  The  minor  pucker 
disclosed  by  making  an  excavation  was  taken  as  being  the  crest 
of  a  great  anticline,  and  as  the  strata  on  both  sides  were 
entirely  argillaceous,  and  gave  no  evidence  at  all  that  could  be 
considered  reliable,  the  error  survived  for  a  long  time,  till  field 
work  in  neighbouring  districts  proved  the  structure  to  be 
entirely  different.  But  for  this  one  unfortunate  excavation  no 
mistake  would  have  been  made. 

In  all  field  work  in  forest  ground,  as  soon  as  the  general 
structure  has  been  ascertained,  the  more  detailed  mapping  of 
stream  sections  should  be  undertaken  with  a  view  to  getting  as 
accurate  an  estimate  as  possible  of  the  thickness  of  strata 
exposed,  and  making  sure  of  the  horizons  of  any  oil-bearing 
strata  that  have  been  discovered.  Should  an  anticlinal  or  dome 
structure  with  gently-dipping  flanks  be  indicated,  the  following 


FIELD   WORK  173 

of  outcrops  well  down  the  flanks  should  be  attempted  before 
the  inner  and  central  portion  is  attacked.  By  this  means 
faults  will  be  more  easily  recognized  and  the  structure  will  be 
more  clearly  and  certainly  delineated,  and  with  less  chance  of 
error  than  if  the  lower  zones  exposed  nearer  to  the  crest  are 
examined  first.  Where  a  very  sharp  flexure  is  indicated  it  will 
be  best  to  follow  and  map  the  crest  first,  as  by  this  means  any 
pitches  of  the  flexure  that  may  be  present  should  be  detected 
and  the  relation  of  surface  indications  to  the  crest  will  be  made 
clear.  Afterwards,  prominent  beds  on  either  flank  can  be 
selected  and  their  outcrops  traced,  and  if  possible  correlated  on 
the  two  sides.  Where  anticlines  are  sharp,  it  is  obvious  that 
the  position  of  the  crest  is  the  most  important  matter,  and  its 
trend  must  be  mapped  as  carefully  as  possible,  while  where  dips 
are  gentle  and  flexures  broad  and  comparatively  speaking  flat, 
the  general  form,  asymmetry  or  pitches  are  of  much  more 
importance  than  any  mapping  of  a  crestal  line,  which,  at  the 
best,  can  only  be  marked  approximately. 

Lateral  variations,  which  may  be  the  cause  of  considerable 
difficulty  in  bare  and  open  ground,  become  much  more  serious 
troubles  to  the  geologist  in  heavily-wooded  country,  but  if  the 
area  be  examined  systematically,  a  general  idea  at  least  of  such 
variations  should  be  obtained.  Correlations  cannot  always  be 
established  with  certainty,  and  the  field-student  must  not 
expect  to  be  able  to  correlate  the  two  sides  of  any  anticline  in 
detail.  The  subdivision  of  the  series  into  groups  may  even  be 
impossible  in  some  cases,  except  locally,  but  the  attempt  to 
subdivide  should  always  be  made ;  the  construction  of  a  new 
road  through  the  forest  may  eventually  furnish  excellent 
evidence  in  side  cuttings,  and  may  enable  a  correlation  that 
has  been  commenced  to  be  carried  to  completion  and  settled 
beyond  doubt.  The  mapping  of  any  bed  locally,  even  if  it 
cannot  be  carried  far,  is  always  advisable,  but  the  extension  of 
dotted  lines,  indicating  uncertainty  as  to  an  outcrop,  between 
the  points  where  the  mapping  of  outcrops  has  terminated,  when 
it  has  not  been  proved  that  the  outcrops  represent  the  same 
horizon,  is  to  be  deprecated. 

Generalizations  on  insufficient  evidence  are  above  all  things 
to  be  avoided ;  it  is  better  to  leave  points  with  regard  to  cor- 
relation unsettled,  and  to  say  so  definitely  when  reporting,  than 
to  force  evidence  to  support  a  conclusion,  however  brilliant, 


1/4  OIL-FINDING 

which  is  not  absolutely  certain.  In  cases  where  there  is  some 
doubt  as  to  the  meaning  of  such  evidence  as  has  been  collected, 
a  doubt  that  leaves  the  geological  structure  a  matter  of  un- 
certainty, a  process  of  elimination  should  be  employed,  and 
every  structure  possible  in  the  particular  circumstances  tried 
and  tested  both  by  map  and  section.  It  will  always  be  possible 
to  reduce  possible  explanations  to  two  or  three,  and  the  ground 
must  not  be  quitted  till  sufficient  evidence  has  been  obtained 
to  enable  the  geologist  to  decide  as  to  which  explanation  is 
the  true  one.  From  the  map,  whether  completed  or  only  half 
finished,  the  various  possible  explanations  can  be  deduced, 
but  it  may  be  necessary  to  return  again  and  again  to  certain 
parts  of  the  area  to  settle  points  which  will  tilt  the  balance 
towards  one  or  other  of  two  alternative  readings  of  the 
geological  structure.  No  mistake  in  structure  is  allowable, 
and  none  should  be  possible  if  reliable  methods  be  employed 
in  the  survey. 

Ratio  of  Boundary  to  Area. — In  all  mapping,  whatever  be 
the  nature  of  the  ground,  it  is  from  the  number  of  miles  of 
geological  lines  drawn  that  we  get  the  clearest  idea  of  the 
efficiency  of  the  geological  survey.  The  area  of  land  surveyed 
in  a  given  time  is  no  test  of  the  ability  of  the  geologist,  but  the 
ratio  of  linear  miles  of  geological  boundary  lines  drawn  to  the 
square  mile  of  area  mapped  shows  at  a  glance  whether  evidence 
has  been  scanty  or  not,  and  is  the  most  certain  criterion  of  the 
care  with  which  the  mapping  has  been  done.  This  ratio  may 
vary  from  fifty  or  sixty  miles  of  boundary  to  one  square  mile 
of  area,  in  very  complicated  and  well-exposed  country,  to 
perhaps  two  to  one  in  obscure  and  wooded  ground.  In  the 
simple  geological  work  of  a  Tertiary  oilfield  the  ratio  will 
seldom  rise  above  7  to  1.  From  400  to  500  miles  of  geological 
lines  represents  a  good  year's  work  for  any  geologist,  allowing 
time  for  the  necessary  indoor  work,  and  it  will  be  found  that 
this  will  hold  good  in  any  country  and  in  any  kind  of  ground, 
bare  or  forest-grown. 

To  sum  up,  in  ground  thickly-clothed  with  vegetation  the 
geologist  must  often  be  content  with  a  map  by  no  means 
complete  or  accurate.  Mistakes  in  accuracy  will  doubtless  be 
made  in  the  mapping  and  need  never  be  worried  over,  so  long 
as  no  error  is  made  with  regard  to  structure.  Should  active 
development  work  follow  the  geological  prospecting  of  an  area, 


FIELD   WORK  175 

details  of  mapping  can  always  be  corrected  as  the  ground  is 
opened  up  and  new  sections  on  roads  and  in  excavations  on 
sites  for  tanks  and  buildings  are  laid  bare.  The  map  can 
always  be  added  to  and  improved  in  details,  but  if  the  structure 
be  incorrectly  delineated,  the  responsibility  for  the  opening  up 
of  a  field  expensive  to  work  and  incapable  of  yielding  results  of 
commercial  importance  may  lie  at  the  door  of  the  geologist. 
Thus,  it  is  not  till  there  is  no  doubt  whatever  about  the 
geological  structure  that  the  geologist  has  any  right  to  speak 
favourably  or  unfavourably  of  any  new  field :  by  advocating 
development  work  without  knowing  what  is  to  be  tested  by 
the  drill,  or  why,  the  geologist  will  class  himself  with  the 
wild-cat  drillers  of  a  former  generation  or  the  company-pro- 
moting experts  from  whom  the  commercial  world  and  the 
unfortunate  public  have  suffered  only  too  severely  and  too  long. 

It  is  naturally  in  thickly-wooded  country,  where  at  the  best 
little  can  be  known  till  development  work  has  begun,  that  the 
greatest  probability  of  ill-advised  speculation  is  afforded,  and 
consequently  the  more  obscure  the  geological  structure  and 
features,  the  more  cautious  the  geologist  must  be  in  making  up 
his  mind  on  the  problems  before  him,  and  the  more  certain 
must  he  be  of  the  main  facts  before  he  dare  venture  upon 
writing  a  report. 

To  visit  a  few  oil-shows,  to  dig  a  few  pits  in  search  of 
evidence,  and  to  make  a  few  observations  of  strike  and  dip  may 
suffice  for  some  experts,  but  no  one  whose  ambition  is  to  take 
rank  as  a  geologist  can  afford  to  advise  a  commercial  company 
upon  the  results  of  what  are  merely  preliminary  observations. 
If  the  area  be  tested  and  failure  attend  the  attempts  to  strike 
oil,  to  shelter  oneself  behind  the  alleged  capricious  nature  of 
that  liquid  is  merely  to  call  attention  to  the  uncertainty  of 
one's  own  field  work,  and  the  unreliability  of  one's  own  mental 
processes. 


CHAPTER  X 

(Fen  BEGINNERS) 
INDOOR  WOEK 

THOUGH  it  is  in  the  field  that  the  real  work  of  the  geologist  is 
done,  systematic  and  careful  indoor  work  must  follow  if  the 
full  fruits  of  his  toil  are  to  be  garnered.  In  the  last  chapter 
the  author  has  endeavoured  to  explain  the  methods  that  he  has 
found  most  effective  in  field-work  under  different  conditions;  it 
remains  to  indicate  the  lines  upon  which  the  necessary  indoor 
work  can  be  conducted  with  the  greatest  facility,  in  the  hope 
that  the  field  student  may  find  in  them  some  hints  that  will 
prove  useful  to  him  in  the  more  irksome  but  no  less  important 
portion  of  his  task. 

When  the  field  work  in  any  district  has  been  completed 
there  must  be  a  gathering  together  and  correlation  of  facts,  a 
reviewing  of  evidence,  and  a  preparation  of  finished  maps  and 
sections,  all  of  which  can  be  done  much  more  effectively  in 
some  office  or  headquarters,  where  there  are  greater  facilities 
and  better  appliances  for  indoor  work  than  the  geologist  will 
be  able  to  carry  with  him  in  the  field,  however  elaborate  his 
equipment. 

As  a  rule  it  will  be  found  that  two  months  of  actual  field 
work,  during  which  an  area  of  from  twenty  to  fifty  square 
miles,  according  to  the  nature  of  the  ground,  should  have  been 
mapped,  will  necessitate  three  weeks  of  indoor  work.  The 
author  has  found  that  this  proportion  of  indoor  work  to  field 
work  holds  good  both  in  bare  ground  where  twenty  or  thirty 
linear  miles  of  geological  lines  are  mapped  in  a  square  mile  of 
area,  and  in  obscure  or  densely  forested  land  where  the  ratio  of 
boundary  to  area  is  2  or  3  to  1. 

Preparation  of  Map. — The  first  thing  to  be  done  is  to 
prepare  the  finished  map  of  the  area  on  the  scale  upon  which 

176 


INDOOR   WORK  177 

the  field  work  has  been  undertaken.  This,  if  there  are  many 
corrections  to  be  made  for  errors  in  traverses  by  dead  reckoning, 
will  be  a  matter  requiring  considerable  care,  and  it  may  be 
necessary  in  order  to  fit  the  traverses  together  with  accuracy 
to  make  a  rough  copy  of  the  map  first.  If  the  area  proves  to 
be  of  little  importance,  or  if  the  evidence  collected  is  insuffi- 
cient to  make  a  large-scale  map,  a  reduction  to  the  one-inch 
scale  may  be  expedient.  In  all  preliminary  work  a  map  on 
the  scale  of  one  inch  to  the  mile  is  generally  quite  sufficient 
to  give  a  clear  idea  of  the  structure  and  the  prospects  of 
obtaining  a  production  of  oil.  Again,  where  a  large  area  has 
been  prospected  on  the  one-inch  scale  in  search  of  localities 
worthy  of  more  careful  examination,  the  smaller  scale  is  quite 
sufficient.  Bat  if  the  area  is  to  be  exploited  and  active 
development  work  is  to  follow  the  geological  examination,  a 
large-scale  map  is  necessary,  even  though  it  may  not  be  possible 
to  put  much  evidence  upon  it  as  the  result  of  the  first  geological 
survey. 

In  the  finished  map  it  may  be  necessary  to  omit  much 
detail  that  has  occupied  a  considerable  time  in  mapping.  To 
introduce  detailed  work  where  it  is  not  essential  will  have  the 
effect  of  confusing  those  who,  having  little  technical  knowledge 
of  geology,  may  yet  have  to  study  the  map  and  master  its 
significance.  The  map  should  be  as  simple  and  clear  as 
possible.  The  strata  should  be  grouped  and  coloured  dis- 
tinctively, so  that  every  essential  point  in  the  geological 
structure  is  brought  out.  '•'  Colour  without  line  "  is  not  allow- 
able ;  that  is  to  say,  every  group  distinguished  by  a  separate 
colour  must  have  a  clearly  defined  boundary  up  to  which  the 
colour  is  brought.  Mapped  lines  of  outcrops  without  special 
colour  may  be  introduced  locally  in  the  midst  of  any  group 
if  any  object  is  to  be  gained  thereby,  such  as  showing  sudden 
changes  of  dip  or  explaining  the  broadening  or  narrowing  of 
outcrops  owing  to  the  contours  of  the  surface.  Similarly  it 
may  be  expedient  to  map  a  fossiliferous  horizon  in  some  group, 
without  colouring  it  specially.  Where  dips  are  gentle  the 
groups  of  strata  coloured  must  be  comparatively  thin,  but  in 
an  area  where  the  rocks  are  highly  inclined  it  is  not  necessary 
to  colour  specially  more  than  three  or  four  groups,  and  they 
may  be  of  considerable  thickness. 

Dip  arrows  and  the  conventional  geological  symbols  should 

N 


i;8  OIL-FINDING 

not  be  distributed  too  thickly  about  the  map.  There  must  be 
enough  to  make  the  structure  clear  to  any  one  without  an 
intimate  acquaintance  with  geological  work,  and  any  line  of 
section  to  which  special  reference  is  to  be  made  should  have 
a  large  number  of  dips  noted,  but  the  map  must  not  be  over- 
loaded with  such  symbols.  It  will  be  found  advisable  to  use 
some  characteristic  and  prominent  symbol  for  surface  indica  - 
tions  of  petroleum,  and  if  the  map  be  on  a  sufficiently  large 
scale  the  words  "gas,"  "oil-seepage,"  "asphalt,"  "inanjak,"  or 
"ozokerite,"  as  the  case  may  be,  can  be  written  or  printed 
beside  the  symbol.  The  author  has  always  used  a  diagonal 
cross  to  mark  surface  indications,  making  it  rather  larger  and 
more  prominent  than  the  symbols  indicating  the  inclination  of 
strata.  A  symbol  indicating  the  direction  of  the  pitch  of  a 
flexure  is  often  useful. 

Every  map  should  be  accompanied  by  a  tablet  showing  the 
groups  of  strata  with  their  distinctive  colours  and  their  order  of 
deposition,  and  all  symbols  used. 

True  north  should  be  shown  on  every  map,  but  it  is  not 
necessary  to  indicate  magnetic  north. 

Sections. — When  the  map  has  been  completed  it  is  often 
useful  and  sometimes  essential  to  make  one  or  more  horizontal 
sections  through  the  area.  These  cannot  be  made  till  the  map 
is  finished.  They  are  very  valuable  as  giving  an  idea  of  the 
structure  to  those  who  are  not  capable  of  reading  a  geological 
map,  though  they  are  not  necessary  to  the  experienced 
geologist. 

It  is  a  common  mistake  of  the  amateur  or  the  untrained 
geologist  to  draw  sections  through  a  property  or  concession 
without  making  a  geological  map  at  all.  Such  sections,  though 
interesting  as  giving  evidence  of  the  ideas  of  their  authors  as  to 
the  geological  structure  of  the  area,  are  generally  useless,  and 
are  almost  invariably  misleading.  Till  the  area  has  been  care- 
fully mapped  the  drawing  of  horizontal  sections  with  any 
measure  of  accuracy  is  practically  impossible. 

Horizontal  sections  should  always  be  drawn  on  the  same 
scale  as  the  map,  and  except  in  very  rare  instances  for  special 
purposes  the  vertical  and  horizontal  scales  should  be  the  same ; 
for  it  is  obviously  impossible  to  give  the  true  dip  and  thickness 
of  strata,  or  the  true  hade  of  the  axis  of  a  fold,  if  the  vertical 
scale  be  different  from  the  horizontal. 


INDOOR   WORK 


179 


In  making  a  horizontal  section  the  contour  of  the  surface 
must  first  be  sketched  from  aneroid  readings,  topographical 
surveys,  or  any  other  evidence  that  is  available.  If  there  are 
no  ascertained  data  to  go  upon,  the  surface  must  be  sketched 
by  guess-work.  Except  in  very  hilly  ground  errors  will  have 
very  little  effect,  as  the  depths  beneath  the  surface  that  will 
have  to  be  considered  will  probably  be  very  much  greater  than 
the  irregularities  of  the  surface,  and  will  make  the  latter  appear 
quite  insignificant. 

A  base  line  is  then  drawn  at  a  sufficient  distance  below  the 
line  representing  the  surface ;  there  is  no  reason  why  this  base 
line  should  be  made  to  coincide  with  sea-level  or  any  height 
above  or  depth  below  it.  Then  from  the  line  of  section  as 


A  45° 


Bso° 


C65' 


D25°         £20° 


FIG.  ISA.— Wrong  method  in  section  drawing. 


FIG.  18B.— Right  method. 

drawn  on  the  map  the  positions  of  geological  boundary  lines 
and  dips  of  strata  as  noted  are  marked  on  the  base  line  and 
projected  to  meet  the  line  representing  the  surface  of  the  ground. 
The  angles  of  dip  are  then  drawn  upwards  from  the  surface 
and  not  downwards  (Fig.  18).  The  reason  for  this  is  that  at  the 
surface  where  the  dips  are  noted  the  angles  of  inclination  are 
only  observed  for  an  infinitesimal  distance.  The  first  thing 
that  one  learns  in  drawing  horizontal  sections  to  scale  is  that 
all  inclined  strata  are  parts  of  great  curves,  and  that  the  dip  of 
no  bed  continues  for  any  considerable  distance  downwards  with- 
out changing.  The  lines  representing  the  bedding  planes  are 
then  continued  downwards,  care  being  taken  to  keep  the  thick- 
ness of  each  group  constant,  unless  variations  in  thickness  have 
been  actually  proved  to  exist.  It  will  be  found  at  once  that 


i8o  OIL-FINDING 

dips  as  observed  are  almost  invariably  too  high  to  make  the 
drawing  of  a  section  an  easy  matter,  and  that  if  there  be  no 
faulting  and  dislocation  of  the  strata  the  minimum  observed 
dips  will  have  to  be  accepted.  This  is  to  some  extent  a  con- 
cession to  convention,  but,  that  notwithstanding,  it  throws  a 
striking  light  upon  the  errors  into  which  one  may  fall  by  a 
blind  acceptance  of  the  dips  observed  at  the  surface  as  being 
constant  for  over  large  distances  downwards,  and  the  danger  of 
depending  on  a  few  observations  of  dip  for  the  elucidation  of 
structure.  It  is  obvious  that  when  it  comes  to  the  measuring 
of  the  thicknesses  of  groups  of  strata  and  calculating  the  depth 
to  oil-bearing  horizons  throughout  a  field,  errors  made  by  noting 
maximum  dip  may  be  sufficient  to  detract  in  no  small  measure 
from  the  practical  value  of  one's  work. 

There  are  naturally  many  details  in  a  section  which  must 
be  almost  purely  imaginary,  and  such  points  as  the  under- 
ground courses  of  unconformable  junctions  and  fault  planes 
cannot  be  indicated  with  certainty  unless  there  is  direct 
evidence  from  boring  journals  to  assist  the  geologist. 

The  horizontal  sections,  if  carefully  constructed,  will  be  of 
great  value  in  checking  the  thicknesses  of  groups  as  obtained 
by  measurement  on  the  map,  but  the  use  of  a  section  is  rather 
to  explain  the  structure  to  those  who  have  difficulty  in  reading 
geological  maps  than  to  give  data  for  the  precise  development 
work  in  an  oilfield.  When  evidence  from  a  number  of  oilwells 
is  available,  sections  can  be  made  on  a  much  larger  scale  than 
that  used  in  mapping,  and  every  petroliferous  horizon  can  be 
shown  at  its  true  depth  ;  the  field-manager  will  then  be  able  to 
adapt  his  methods  to  the  end  in  view  in  each  well,  knowing 
exactly  at  what  depths  water  must  be  shut  off  and  where  oil  is 
likely  to  be  struck.  In  new  untested  fields  such  accuracy  is 
unfortunately  very  seldom  possible. 

Vertical  Section. — After  the  horizontal  section  has  been 
completed  it  is  often  expedient  to  construct  a  vertical  section 
of  all  the  strata  exposed  in  the  area,  leaving  room  below  for  the 
strata  to  be  proved  in  the  drilling.  The  vertical  section  must 
be  drawn  to  scale,  but  a  much  larger  scale  should  be  employed 
than  that  on  which  the  ground  has  been  mapped.  The  groups 
of  strata,  coloured  as  on  the  map  and  in  their  relative  thick- 
nesses, will  be  marked  clearly  in  the  vertical  section,  and  the 
horizons  of  all  fossiliferous  beds  and  oil-bearing  bands  observed 


INDOOR   WORK  181 

will  be  noted  as  accurately  as  possible.  It  is  advisable  also  to 
mark  the  initial  horizons  of  any  wells  that  have  been  drilled, 
or  that  it  is  proposed  to  drill,  so  that  it  will  at  once  be  apparent 
what  horizons  have  been  or  can  be  tested. 

Palaeontological  Work. — Any  fossil  evidence  that  has  been 
collected  must  then  be  gone  over  and  compared  with  previous 
collections  or  books  of  reference  in  order  that  any  organisms 
of  importance  in  establishing  stratigraphical  horizons  may  be 
recognized.  Where  palaeontological  evidence  is  abundant,  as 
for  instance  in  Burma,  some  such  method  as  that  described  in 
Chapter  VII.  may  be  made  use  of,  but  as  a  rule  a  less  elaborate 
system  will  be  quite  adequate ;  it  is  seldom  that  fossil  evidence 
becomes  of  any  great  importance  till  a  great  mass  of  material 
has  been  collected. 

Petrographical  Work. — It  is  but  rarely  that  petrographical 
work  is  of  much  value  in  an  oilfield  till  after  it  has  been  at 
least  partially  developed,  but  there  are  often  points  that  can 
be  settled  by  the  use  of  a  microscope,  and  that  may  eventually 
prove  of  vital  importance.  The  examination  of  oilsands  may 
furnish  very  valuable  evidence,  as  it  is  often  possible  to  identify 
different  sands  by  their  mineral  contents.  This  is  especially 
important  when  an  unconformability  is  suspected  but  has  not 
been  proved.  Sands  that  appear  very  similar  may  be  from 
formations  of  different  ages,  and  may  contain  minerals  which 
enable  them  to  be  distinguished  at  once ;  the  heavy  minerals 
are  frequently  the  most  useful  in  this  respect.  The  presence 
of  Kaolin  or  decomposed  felspar  may  be  a  point  of  great  im- 
portance, as  in  some  parts  of  Burma  in  separating  post  from 
pre-volcanic  strata. 

The  determination  of  the  extent  to  which  a  limestone  has 
been  dolomitized  is  another  question  that  may  be  of  vital  impor- 
ance  in  oilfields  where  the  petroliferous  rocks  are  calcareous.  All 
these  matters  can  be  dealt  with  by  means  of  a  petrographical 
microscope  without  the  necessity  of  making  any  chemical  tests, 
and  though  that  instrument  can  hardly  be  regarded  as  an 
essential  part  of  the  petroleum-geologist's  equipment,  it  may 
be  of  very  great  use  when  other  evidence  fails  and  only  petro- 
logical  work  can  be  depended  on  to  solve  some  difficult  problem. 
There  is,  in  fact,  no  department  of  geological  work  that  cannot 


182  OIL-FINDING 

in  certain  circumstances  be  brought  to  the  aid  of  the  geologist 
who  is  engaged  in  the  study  of  oilfields. 

Report  Writing. — With  the  completion  of  any  palaeonto- 
logical  or  petrographical  work  that  may  have  had  to  be  under- 
taken, the  geologist's  task  is  practically  over  for  the  time  being ; 
it  only  remains  to  write  a  report  upon  the  area  examined.  It 
is  in  the  field  work  and  the  preparation  of  map  and  sections 
that  the  real  work  of  the  geologist  has  been  accomplished,  but 
by  a  very  natural  irony  it  is  the  report  that  will  receive  the 
most  attention,  and  the  young  geologist  may  be  assured  that 
for  one  man  who  will  study  his  maps,  ten  at  least  will  read  his 
reports  and  interpret  them  in  their  own  fashion.  Chairmen  of 
Companies,  Managing  Directors,  Technical  Experts,  Company 
Promoters,  and  even  a  small  section  of  the  shareholders  and 
the  speculative  general  public  all  attach  value  to  a  report  rather 
than  a  geological  map,  and  consequently  it  is  essential  that 
great  care  should  be  taken  in  the  writing  of  it.  As  with  most 
practical  geologists,  among  whom  the  writer  has  no  further 
ambition  than  to  be  classed,  the  hammer  is  mightier  than  the 
pen,  the  writing  of  the  necessary  reports  may  be  not  only 
difficult  but  irksome. 

In  the  report  on  a  new  area,  a  presumed  but  untested  oil- 
field, brevity  is  the  first  essential.  The  geologist,  if  he  has 
sufficient  time,  should  write  out  his  report  three  times,  each 
time  making  it  shorter  by  cutting  out  all  that  does  not  seem 
absolutely  necessary.  Looked  at  from  this  point  of  view  it  is 
wonderful  how  much  "padding"  can  be  detected  in  even  a 
workman-like  and  concise  report. 

Clearness  is  no  less  essential.  Technical  geological  terms 
should  be  eschewed  as  far  as  possible,  as  it  is  probable  that  of 
those  who  read  a  report  few  will  have  more  than  a  smattering 
of  geological  knowledge.  It  is  not  difficult  to  explain  in  simple 
language  all  that  can  be  conveyed  by  sesquipedalian  scientific 
phraseology.  Again,  it  is  not  enough  that  the  writer  is  clear  in 
his  own  mind  upon  a  point ;  he  must  set  it  down  so  that  the 
reader  cannot  fail  to  be  clear  in  his  mind  as  to  what  is  meant 
to  be  conveyed.  This  is  not  such  a  simple  matter  as  it  appears 
at  first  sight.  In  correspondence  with  reference  to  a  report  or 
the  ground  with  which  it  deals,  the  geologist's  statements  will  be 
paraphrased  and  unintentionally  misquoted,  and  some  day  a 
statement  which  the  writer  considered  impossible  to  misconstrue 


INDOOR   WORK  183 

will  come  back  to  him  distorted  out  of  all  recognition  and 
labelled  as  his  opinion.  Therefore  short,  crisp  sentences,  with- 
out conditional  clauses,  should  be  the  rule. 

Graces  of  style  and  the  neat  turning  of  phrases  are  to  be 
avoided;  it  is  possible  to  give  a  literary  flavour  to  scientific 
work,  as  many  of  the  greatest  geologists,  from  Hugh  Miller 
onwards,  have  taught  us,  but  it  is  not  literature  that  is  required 
from  the  field  geologist,  but  facts.  If  in  reading  over  the 
draft  of  a  report  one  comes  upon  any  sentence  with  which  one 
is  particularly  pleased,  the  wisest  course  is  to  cut  it  out  at  once. 
Be  literal  rather  than  literary. 

The  point  most  essential  of  all  is  to  stick  to  facts.  Opinions 
must  not  be  given  on  any  points  of  importance  in  the  geology 
of  the  area  examined.  It  is,  of  course,  impossible  to  avoid 
giving  an  opinion  upon  such  a  question  as  whether  an  area  is 
sufficiently  promising  to  warrant  development  work  being  under- 
taken or  not,  but  in  dealing  with  questions  of  structure,  lateral 
variation,  thickness  of  oil-bearing  strata,  depth  to  be  drilled,  etc., 
no  mere  opinion  will  suffice.  If  the  certified  facts  cannot  be 
given,  the  geologist  must  say  so  clearly.  "  To  the  best  of  my 
belief,"  "as  far  as  I  could  ascertain,"  "in  my  opinion,"  "it 
seems  to  me,"  and  the  numberless  similar  phrases  should  be 
tabooed.  Indeed,  the  geologist  will  do  well  to  shun  the  use  of 
the  first  personal  pronoun  as  much  as  possible,  and  to  write  his 
report  in  the  third  person.  The  report  will  read  better  and 
will  appeal  more  forcibly  to  both  scientific  and  commercial 
readers  if  the  writer  does  not  intrude  his  personality,  but  allows 
the  facts  as  ascertained  by  him  and  set  forth  in  map,  section 
and  report  to  speak  for  themselves. 

The  ideal  report  must  be  partly  descriptive ;  it  must  explain 
the  map  to  those  who  may  not  be  able  to  read  geological  maps. 
It  must  call  attention  to  the  points  of  greatest  importance  in 
the  structure,  etc.,  but  it  is  quite  unnecessary  to  describe  and 
explain  the  map  in  detail.  Geological  structure  can  be  dealt 
with  very  briefly:  the  map  and  sections  should  be  sufficient 
with  a  few  sentences  of  explanation.  Enough  must  be  written 
concerning  the  methods  of  mapping  employed  and  the  nature 
of  the  strata  examined  to  show  the  care  with  which  the  survey 
has  been  conducted.  The  distinguishing  characteristics  of 
different  groups  of  strata  mapped  must  be  mentioned,  but  long 
lithological  descriptions  are  unnecessary. 


184  OIL-FINDING 

Evidence  of  the  presence  of  petroleum  should  be  treated 
separately  and  at  greater  length,  for  much,  and  in  some  cases 
perhaps  undue,  importance  will  be  attached  to  such  evidence 
by  those  for  whom  the  report  is  written.  It  is  always  necessary 
to  prove  as  conclusively  as  possible  the  petroliferous  nature  of 
the  series  that  has  been  studied  geologically,  and  the  conditions 
under  which  .surface  shows  of  petroleum  occur  afford  very 
valuable  hints  to  the  expert  or  technical  adviser  and  the  field 
manager. 

A  comparison  of  the  field  with  other  areas  as  regards 
structure,  stratigraphy,  and  surface  indications,  especially  if 
those  other  areas  are  producing  fields,  may  be  introduced  with 
advantage  in  this  section  of  the  report  in  order  to  give  some 
idea  of  the  significance  of  the  evidence,  but  any  canvassing 
of  the  probabilities  of  proving  a  valuable  field  is  better  kept 
for  the  final  section. 

It  is  always  best  to  divide  a  report  into  clearly  defined 
sections,  and  to  keep  each  piece  of  evidence  rigidly  to  its  own 
section.  These  sections  may  again  be  subdivided,  and  the 
report  should  be  headed  by  a  page  showing  the  divisions  and 
subdivisions,  so  that  any  part  can  be  referred  to  with  the  least 
trouble  and  delay,  A  convenient  form,  which  the  writer  has 
found  to  meet  most  cases  of  new  and  untested  fields,  is  as 
follows  :— 

Report  on  Concession. 

I.  Introductory. 
II.  Formations  and  Strata. 

III.  Geological  Structure. 

IV.  Oilshows. 

V.  General  Conclusions  and  Recommendations. 
Fig.  1.  Map  of  Concession  and  surroundings  6  in.  to  the  mile. 
Fig.  2.  Horizontal  Section  .        »        .         .  do. 

Fig.  3.  Vertical  Section     ,        .       -V        .     2  in.  to  100  feet. 

In  the  first  section  the  position  of  the  property  or  concession 
is  briefly  described,  and  the  nature  of  the  ground,  whether  low 
or  hilly,  forested  or  bare.  The  methods  of  survey  employed 
are  explained  and  the  work  of  any  previous  observers  in  the 
same  area  must  be  touched  on. 

In  the  second  section  the  various  formations  exposed  in  the 


INDOOR   WORK  185 

area  are  described  shortly  in  their  stratigraphical  relations. 
Each  group  of  strata  mapped  and  coloured  separately  is  described 
and  its  thickness  estimated,  and  the  horizons  of  oil-bearing 
strata  and  fossiliferous  beds  are  given.  If  fossil  evidence  be 
very  abundant,  it  is  better  not  to  give  it  at  length  in  this 
section,  but  to  state  the  general  conclusions  arrived  at  from 
palaeontological  work,  and  keep  a  detailed  account  of  it  for  an 
appendix  to  the  report. 

In  Section  III  the  structure  as  shown  by  the  map  and 
horizontal  section  is  explained  briefly,  and  the  account  may  be 
subdivided  into  evidence  of:  (1)  Flexuring;  (2)  Faulting,  and 
(3)  Unconformabilities,  etc.  as  may  be  necessary. 

A  special  section  upon  the  indications  of  petroleum  is  only 
necessary  when  they  are  extensive  and  important  enough  to 
deserve  careful  description.  If  the  "oilshows"  are  few  and 
insignificant  this  section  can  be  merged  in  Section  II.  ' 

In  the  last  section  the  general  conclusions  on  scientific 
points  must  be  stated  very  clearly  and  briefly:  it  is  often 
advisable  to  number  them,  e.g. : — 

(1)  The  strata  are  of  the  nature  common  to  many  oilfields, 

and  give  evidence  of  containing  petroleum  at  intervals 
throughout  a  thickness  of  3000  feet. 

(2)  The  geological  structure  over  the  greater  part  of  the 

area  is  unfavourable  to  a  production  of  petroleum,  but 
in  the  north-west  corner  of  the  concession  is  very 
favourable. 

(3)  The  area  of  favourable  structure  is  approximately 

acres,  etc.,  etc. 

Though  the  scientific  reader  will  doubtless  give  full 
attention  to  the  whole  report,  it  is  the  last  section,  the  "  con- 
clusions and  recommendations"  that  will  be  studied  most 
closely,  and  that  will  be  quoted  and  canvassed  by  every  one 
else;  indeed,  the  earlier  part  of  the  report  may  merely  be 
glanced  through. 

After  the  "  conclusions  "  comes  the  "  opinion  "  as  to  whether 
development  work  on  the  new  field  will  be  justified  or  not.  If 
properly  led  up  to,  this  opinion  should  appear  inevitable. 

Then,  if  a  favourable  opinion  has  been  given,  come  the 
recommendations  as  to  how  the  area  should  be  developed.  The 
sites  chosen  for  test-wells  should  be  described,  and  the  reasons 


1 86  OIL-FINDING 

for  selecting  them  given.  If  locations  have  actually  been 
marked  on  the  ground  and  on  the  map,  it  is  not  necessary  to 
deal  at  length  with  their  advantages  and  disadvantages;  the 
initial  horizon  of  each  test-well  can  be  shown  on  the  vertical 
section,  and  the  position  of  each  as  regards  geological  structure 
can  be  given  on  the  horizontal  section. 

The  depth  to  be  drilled  in  each  case  should  be  stated,  as 
well  as  the  nature  of  the  strata  to  be  drilled  through,  and  any 
difficulties  likely  to  be  encountered  in  the  drilling  through 
the  occurrence  of  water-sands,  loosely-compacted  sands,  thick 
soft  clays,  or  steeply-dipping  strata  must  be  mentioned. 

Proximity  to  water  supply,  best  means  of  access  to  the  well 
sites,  and  difficulties  in  the  way  of  road-making  incidental  to 
the  nature  of  the  country  and  strata  should  be  touched  upon  : 
though  these  matters  are  hardly  within  the  province  of  the 
geologist,  any  information  about  them  will  be  of  value  to  a  field 
manager. 

Finally,  if  the  geologist  has  sufficient  experience  in  oilfield 
work  to  justify  him  in  so  doing,  the  method  of  drilling  which 
he  believes  will  give  the  most  successful  results  in  the  special 
circumstances,  and  the  expenditure  which  he  considers  should 
be  sufficient  to  allow  of  the  test-wells  being  drilled  in  a 
satisfactory  manner,  may  be  indicated.  On  these  latter  points, 
however,  it  is  well  to  use  a  wise  caution.  Unforeseen  circum- 
stances may  arise  to  falsify  estimates  of  expenditure,  and  it  is 
better,  unless  specially  requested  to  do  otherwise,  to  leave  all 
such  matters  to  those  who  will  have  to  be  responsible  for  the 
practical  development  work. 

It  is  a  very  simple  matter  when  dealing  with  a  new  oilfield 
to  write  a  favourable  report  in  somewhat  indefinite  terms, 
dealing  with  such  evidence  as  has  been  obtained  in  a  general 
way,  and  not  stating  the  reasons  why  any  particular  fact  is 
regarded  as  favourable.  Eeports  of  this  kind  are  very  common 
nowadays,  and  may  frequently  be  found  in  a  prospectus. 
The  geologist  who  wishes  to  establish  his  reputation  for 
reliability  must  be  careful  not  to  fall  into  this  style,  which  is 
fatally  easy  to  acquire.  The  disadvantages  of  a  new  field  should 
be  stated  as  clearly  as  its  advantages,  and  though  the  expert 
who  does  not  hesitate  to  condemn  a  field  upon  evidence  which 
he  gives,  and  holds  to  be  sufficient,  is  never  so  popular  as  he 
who  can  write  a  carefully  safe-guarded  report,  which  at  the 


INDOOR   WORK  187 

same  time  gives  the  reader  a  highly  favourable  impression  of 
the  prospects  of  a  field,  in  the  long  run  the  man  who  confines 
himself  to  the  stating  of  facts,  and  has  the  courage  of  his 
convictions,  will  carry  the  most  weight.  A  reputation  for 
caution  and  even  pessimism  will  be  of  more  value  to  the  young 
geologist  than  an  ill-regulated  enthusiasm  which  may  have  the 
effect  of  inducing  capitalists  and  the  public  to  sink  large  sums 
in  fruitless  expenditure. 

Report  on  a  proved  field. — In  reporting  upon  a  field  already 
tested  and  partially  developed,  the  geologist  has  a  much  more 
complicated  task.  An  accurate  topographical  map  in  all 
probability  will  be  available,  and  the  geological  data  must 
be  noted  upon  it  with  great  care.  A  larger  scale  than  6  or 
8  inches  to  the  mile  will  probably  have  to  be  employed,  and 
the  exact  position  of  every  well,  drilled  or  drilling,  must  be 
marked.  Then  after  the  geological  map  and  horizontal  section 
have  been  completed,  logs  and  boring  journals  must  be  con- 
sulted and  every  well  projected  on  to  the  horizontal  section, 
showing  its  initial  horizon  and  the  depth  reached.  The 
underground  geology  can  then  be  added  from  the  logs  of  the 
wells  and  a  correlation  of  the  oil-bearing  horizons  attempted. 
Where  many  wells  have  been  drilled  it  is  often  possible  to 
correlate  every  water-sand  and  every  oil  or  gas-show  throughout 
a  field,  and  to  draw  contour  lines  upon  the  map  showing  the 
depths  to  an  oil  horizon  at  any  part  of  the  area.  In  some  of 
the  American  fields,  notably  that  Coalinga  in  California,  this 
has  been  done  with  great  success.  When  such  accurate  work 
is  possible  the  required  depth  for  each  new  well  can  be 
calculated  from  the  elevation  of  its  site  and  its  position  with 
regard  to  these  contour  lines,  and  the  depth  at  which  water 
must  be  shut  off  can  be  given  with  certainty,  so  that  a  field 
manager  is  enabled  to  save  much  expense  by  adapting  his 
methods  to  the  particular  object  aimed  at  and  economizing  in 
the  matter  of  casing. 

The  report  will  require  to  be  written  on  a  different  system ; 
there  must  be  a  section  dealing  at  length  with  the  evidence 
from  wells  and  the  correlation  of  the  underground  strata. 
These,  however,  are  matters  not  entirely  geological,  and  can 
be  undertaken  by  persons  without  any  special  technical  know- 
ledge. It  is  before  a  field  has  reached  the  producing  stage 
that  the  services  of  a  geologist  are  essential.  After  the  map 


i88  OIL-FINDING 

and  horizontal  sections  have  been  completed,  and  the  confines 
of  the  field  proved,  the  petroleum  expert  may  take  the  place 
of  the  geologist. 

When  working  in  a  producing  field  great  caution  must  be 
exercised  in  taking  hearsay  evidence  about  the  strata  in  any 
well  and  the  shows  of  water  at  any  horizon  in  it.  The  logs 
of  wells  are  not  always  reliable,  and  even  when  kept  with 
care  too  much  is  often  left  to  the  personal  opinion  of  the 
driller.  Strata  are  frequently  incorrectly  described,  and  two 
drillers  may  give  different  names  to  the  same  type  of  sediment. 
Boring  records  and  hearsay  evidence,  therefore,  must  not  be 
blindly  relied  upon.  Not  that  the  geologist  will  be  intentionally 
misled  by  the  practical  workers  in  an  oilfield — though  cases 
of  deliberate  attempts  to  mislead  the  scientific  worker  are  not 
altogether  unknown — but  the  mind  untrained  in  scientific  work 
may  not  be  able  to  convey  or  express  information  in  such  a 
form  that  it  can  be  grasped  accurately.  From  a  report  hear- 
say evidence  should  be  rigidly  excluded ;  it  is  better  to  leave 
a  point  unsettled  than  to  rely,  however  slightly,  upon  second- 
hand information. 

One  point  with  regard  to  the  writing  of  reports  remains 
to  be  touched  upon.  It  will  frequently  happen  that  the 
geologist  in  the  course  of  his  field  work  will  establish,  or  obtain 
evidence  about,  some  point  of  general  scientific  interest,  and 
he  will  naturally  be  tempted  to  enlarge  upon  it  in  his  report. 
In  such  cases  the  best  procedure  is  to  consider  whether  the 
scientific  point  in  question  is  of  practical  importance  in  the 
commercial  development  of  any  particular  field,  and  whether 
other  members  of  a  scientific  staff  working  in  the  same  interests 
will  be  helped  in  their  investigations  by  the  new  knowledge 
acquired.  If  so,  the  evidence  should  be  described  briefly  and 
the  conclusion  stated.  Otherwise  it  is  better  not  to  overload 
a  report  with  matters,  however  interesting  and  important  from 
the  scientific  point  of  view,  that  have  no  direct  bearing  upon 
the  practical  finding  and  producing  of  petroleum.  Appendices 
can  always  be  written  to  a  report  to  contain  such  results  of 
the  geologist's  investigations  as  are  of  greater  scientific  than 
practical  importance. 

Reports  are  always  subject  to  criticism,  and  as  a  matter  of 
course  always  receive  it,  practical  or  academic,  pertinent  or 
impertinent,  fair  or  unfair,  and  occasionally  merely  ignorant. 


INDOOR   WORK  189 

Any  criticism  is  stimulating,  or  should  be  so,  to  the  practical 
geologist,  and  in  the  majority  of  cases  must  be  beneficial  how- 
soever unfair  it  may  be.  The  answer  to  it  is  in  work  rather 
than  controversy.  Theories  may  be  promulgated,  tested  by  the 
facts,  and  fall ;  fallacies  often  die  very  hard  and  may  even  be 
brought  to  life  again  unexpectedly,  but  the  search  for  truth 
goes  on,  and  the  dealer  in  facts  has  in  the  end  the  victory  over 
the  critic  steeped  in  theory  who  has  not  the  advantage  of  first- 
hand acquaintance  with  all  the  evidence.  Therefore  the  field- 
student  in  his  writings  should  eschew  theory  and  stick  to  facts, 
nor  resent  the  spur  of  criticism  however  clumsily  applied. 

In  these  notes  the  author  is  conscious  that  he  is  setting 
forth,  probably  at  undue  length,  a  great  deal  of  very  obvious 
advice,  which  even  the  tyro  in  geological  work  in  oilfields  may 
stigmatize  as  common-place  and  banal.  "  These  matters,"  he 
may  say,  "are  merely  common  sense,"  in  which  he  neither 
requires  nor  desires  instruction.  The  author  does  not  cavil  at, 
but  rather  applauds  such  a  dictum ;  each  man  must  depend  on 
his  own  common  sense,  and  to  teach  geology  from  books  rather 
than  in  the  field  is  an  academic  absurdity.  Out  of  the  fruits 
of  considerable  experience  the  author  has  written  in  the  last 
two  chapters  these  notes,  not  claiming  for  them  any  originality, 
nor  desiring  to  dogmatize,  but  hoping  that  here  and  there 
among  them  the  beginner  may  find  something  that  will  help 
him  in  his  practical  work. 

It  may  seem  that  the  duties  of  a  petroleum  geologist  have 
been  made  to  appear  samewhat  elaborate  and  complicated. 
They  may  be,  and  indeed  often  are  so ;  the  geological  work  in 
an  oilfield,  especially  in  a  Tertiary  oilfield,  is  in  itself  simple, 
but  to  guide  the  development  work  of  a  petroleum  company 
with  complete  success,  without  causing  needless  expenditure, 
and  without  having  to  admit  failure  now  and  then,  may  be  very 
difficult.  Every  kind  of  evidence  must  be  studied,  every 
precaution  taken,  and  every  detail  examined  if  certainty  is  to 
be  arrived  at.  And  that  in  very  many  instances  practical 
certainty  can  be  attained  in  oil-finding  is  the  firm  belief  of  the 
author,  though  years  of  laborious  fieldwork  and  research  under 
conditions  not  always  of  the  most  attractive  may  have  to  be 
accomplished  before  such  a  result  is  within  sight. 

A  great  field  is  opening  up  nowadays  for  the  prospecting 
geologist,  the  man  trained  in  scientific  processes  of  thought, 


OIL-FINDING 

and  physically  fitted  to  endure  the  hardships  and  discomforts 
of  field  work  in  those  parts  of  the  world  where  nature  is  not  yet 
shackled  by  civilization.  It  is  in  tropical  and  sub-tropical 
countries  that  much  of  the  earth's  richest  stores  are  to  be 
searched  for  and  won,  and  it  is  to  him  who  can  withstand 
unfavourable  climatic  conditions,  under  tropical  sun,  or  in  dark 
forest,  on  desert  and  barren  hill,  or  in  cane-field  and  plantation, 
that  the  prizes  will- fall. 

In  no  branch  of  geological  work  is  there  a  more  promising 
field  than  that  offered  by  the  search  for  petroleum,  and  no  com- 
mercial enterprise  depends  more  for  its  success  upon  the 
geologist  than  the  finding  and  winning  of  oil.  The  life  of  the 
oil-finder,  with  its  travel  in  many  lands,  its  contact  with 
many  races,  and  its  frequent  change  of  scene,  is,  taking  the 
rough  with  the  smooth,  a  thoroughly  enjoyable  one.  To 
the  sportsman — and  every  field-geologist  should  be  somewhat 
of  a  sportsman  at  heart — there  are  moments  that  compensate 
one  richly  for  the  hardships  incidental  to  the  exploration 
of  wild  and  little  known  country. 

If  this  little  introduction  to  the  great  subject  of  oil-finding 
be  instrumental  in  turning  the  attention  of  the  young  geologist 
to  the  fascinating  subject  of  petroleum,  and  be  of  service,  in 
however  slight  a  degree,  in  setting  his  feet  in  the  path  that 
leads  to  success,  the  aim  of  the  author  will  be  accomplished 
and  his  labour  rewarded. 


INDEX 


ABNEY'S  level,  154 

Adsorptive  properties  of  clay,  etc.,  for 
bitumen,  21,  29,  41,  97 

Alaska,  41 

Alluvium,  165 

Alteration  in  character  of  sediment, 
124 

Ammonia  as  evidence  of  animal 
matter,  21 

Ammonium  sulphate  from  oil-shales 
and  from  peat,  27 

Angle  of  dip,  changes  in,  75,  142,  144 

Anglo-Persian  Oil  Company,  47 

Animal  matter,  theories  of  origin 
from,  4  ;  present  in  oil-shales,  21 

Anthracites  and  bituminous  coal, 
origin  of  difference,  20 

Anticlines,  70  ;  asymmetrical,  72, 135, 
137 ;  compound,  73  ;  symmetri- 
cal, 71,  134,  140 

Arakan  Yomas,  16,  63,  83 

Arenaceous  beds,  deposition  of,  50 

cover,  19,  29 

Argiline,  43 

Argillaceous  cover,  19,  29,  30 

matter  and  formation  of  petro- 
leum, 29 

rock  as  a  filter,  42 

Arkoses,  118 

Asphalt  deposits,  89,  93-103 

Asphaltene,  112 

Asphalt  from  Pitch  Lake,  Trinidad, 
51 ;  analysis  of,  95,  96 

Asphaltic  oils,  filtration  by  clay,  29, 
41 ;  as  contrasted  with  paraffin 
oils,  36,  53,  91,  115,  149;  from 
monoclines,  74 

Asmari  limestone,  48,  84 

Asymmetrical  anticlines,  72,  135-139 

BAKU,  40,  145;  oil-wells,  51;  sands, 

51,  52 
Baluchistan,  16,  34,  41,  43,  49,  59,  69, 

122,  160 
Barbados,   3,   18,  108;  manjak,  112; 

tar-sand,  109, 113  ;  unconformity, 

86 

Bassein  Series,  83 
Bitumen,  adsorption  of ,  21 ;  of  Pitch 

Lake,  Trinidad,  51,  95,  96 


Bituminous  compounds,  and  igneous 
action,  3,  25;  from  vegetable 
matter,  12  ;  from  peat,  27 

Bituminous  outcrops  and  impregna- 
tion, 89,  108-110 

"  Blackband,"  12 

Blue  clays,  7 

"  Boiling  Spring,"  Barbados,  108 

Borehole  indications,  115 

Borneo,  20 

Boundary  to  area  ratio,  174 

Brine,  associated  with  petroleum,  32- 

35,  115 

Burma,  2,  15,  16,  68,  69,  74,  160,  181 ; 
Bassein  Series,  83 ;  clay  conglo- 
merates, 62  ;  correlation  of  series 
by  fauna,  125-131;  faults,  78; 
mud-volcanoes,  107 ;  paraffin  oil, 

36,  91,  110;  Prome  Series,  121; 
Sabe  field,  85 ;  unconformity,  81- 
83;    Tertiary     Series,     39,     63; 
Twingon,   149;    vide    Irrawaddy 
Series,  Pegu  Series,  Yaw,  Yenang- 
young,  Yenankyat. 

Burmah  Oil  Company,  15,  125  if,  146 
Burnt  Cliff,  Barbados,  18 

CADMAN,  Professor  J.,  viii.,  18 
Calcareous  cement  and  concretions, 

54,  55,  123 
Californian  oil-fields,   5,   39,  51,  74 

102 
Carbonaceous     phases    passing   into 

petroliferous,  15,  20,  31 

shales,  formation  of,  12,  14,  15 

Carboniferous    measures,    vide    Coal 

seams. 

Carmody,  Prof.,  7,  52,  92,  104,  113 
Cedros,  Trinidad,  17,  19,  55,  105 
Chemical     researches    on    origin    of 

petroleum,  25 
"  Chemin  de  Diable,"  106 
Clapp,  F.  G.,  85 
Clay,  14,  54  ;  Kimeridge,  17,  22 
Clay  conglomerate,  62 
Clay-gall  beds,  62 
Clay-ironstone,  118 
Clifton  sand,  85 
Clinometer,  153,  154 
Coal,  associated  with  petroleum,  20,  21 


191 


1 92 


INDEX 


Coal-seams,  formation  of,  12,  14; 
connected  with  petroleum,  20 

Coalinga,  California,  187 

Columbia  Estate,  105 

Columnar  jointing,  112 

Compass,  prismatic,  153,  157 

Compound  anticlines,  73 

Conglomerates,  oil-bearing,  47 

Contour  of  sand-grain,  51,  97 

Correlation  of  strata,  on  lithological 
grounds,  unsatisfactory,  65,  118  ; 
by  fossil  fauna,  120-130 

"  Cover,"  effect  of,  19,  28-30;  as  con- 
cealing petroleum,  116 

Cretaceous  formation,  41,  58,  66,  169 

"  Crevices,"  44 

Crouliansky,  M.,  viii 

Cunapo  lignite  field,  61 

"  DEATH-MARK,"  8 

Deltas  and  deltaic  conditions,  13,  23, 
59,  118 ;  fossil  evidence  of  direc- 
tion of  formation,  62  ;  sedimenta- 
tion in,  28  ;  vide  Pegu  Series 

Depth  of  well,  calculation  for,  143, 
144 

Depth-temperature,  26,  28,  32 

Desiccation,  34 

"  Devil's  Woodyard,"  106,  114 

Diatoms,  5,  8 

"  Die  Fossilen  von  Java,"  128 

Dip,  angle  of,  75,  142,  144 ;  estima- 
tion of,  163 

Distillation,  local,  from  igneous  or 
volcanic  action,  3,  25 

Dolomites,  47 

Dolomitization  of  limestones  as  affect- 
ing storage,  47,  48 

Dome  structure,  70,  71,  80 ;  location 
of  well  on,  134-137,  140 

Drill,  kind  of,  150 

EARTH-MOVEMENTS,  28,  34,  57,  64; 

their  study,  67-69 
Eastern  States  of  America,  oilfields 

in,  38,  71,  72 
Eldridge,  Mr.,  Ill 
Engler  and  Hofer,  1,  5,  25 
Equipment  for  prospecting,  etc.,  152- 

156 
Estuaries,   sludge    from,  6,  7;    free 

from  seaweeds,  23;    in  Tertiary 

times,  59 

Evolution  of  gas,  vide  Gas  evolution 
Excavations,  170 
Eye  training,  160 

FACHER  or  fan  structure,  68 

Faule  Island,  43 

Fault-fissures,  45 

Faults,  as  the  geologist's  deus  ex 
machina,  44,  70 ;  as  affecting  lo- 
cation of  wells,  145  ;^as  affecting 


storage,  45;  as  part  of  earth- 
movement,  69 ;  their  true  nature 
and  effect,  76-80, 146 

Fauna,  as  aids  to  stratigraphy,  124 ; 
as  evidence  for  animal  origin  of 
petroleum,  8  ;  as  indicating  direc- 
tion of  delta  formation,  62 ; 
example  of  use  from  Burmese 
Tertiaries,  126-130 

Field-mapping,  vide  Map-making 

Filtration  of  oil,  41,  91,  138 

Fish,  as  origin  of  petroleum,  10 

"  Fissures,"  44 

Flexures,  68,  69,  76-80 ;  as  affecting 
well-sites,  135-140;  in  mapping, 
172 

Folds  and  folding,  68,  69,  76-80;  in 
Barbados,  86 

Foraminifera,  5,  8,  39 

Fossil  fauna,  vide  Fauna 

Fucoids,  theory  of  origin  from,  22-24  ; 
Cambrian  beds,  22 

Fyzabad,  Trinidad,  102 

GALEOTA  oil-bearing  group,  91 ;  oil- 
sand,  104 

Galfa  Point,  Trinidad,  55,  107 

Galicia,  41,  139 

Gas  evolution,  89,  101,  103,  117 

Gas-pressure,  38,  40,  117,  134 

Gas-sands,  50 

Gas-shows,  108,  117,  187 

Gasteropods,  8,  62,  128 

Gas  wells,  107 

Geological  Survey  of  Great  Britain, 
viii,  20,  152,  154 

Geological  Survey  of  India,  72,  81, 
126 

Ghasij  shales,  16 

Gilsonite,  45, 110,  111 

Glauconite,  59,  118 

Grahamite,  110 

Grande  Riviere,  16 

Griswold,  W.  T.,  73 

Grits,  oil-bearing,  47,  118 

Guapo,  Bay,  96  ;  Oil  Company,  30 

Guayaguayare,  Trinidad,  108 

Gypsum,  59,  118 

HADE  of  axis,  decrease  and  direction 

of,  137, 139 
"  Hard  shells,"  15 
Harnai  Valley  coal,  49 
Hydrostatic  pressure,  39,  139 
Hypogene  origin,  theory  of,  2 

IGNEOUS  action,  as  causing  distilla- 
tion, 3,  25 

Impregation  of  rocks,  46,  47 
Indications  in  a  borehole,  ]  15 
Inorganic  origin,  theories  of,  2 
Intrusion  of  veins  of  manjak,  45 
Irois,  Trinidad,  18 


INDEX 


193 


Irrawaddy,  13,  64;  Series,  69,  78, 
unconformable  with  Pegu  Series, 
81,  83,  125 

JAMES,  S.  Lister,  viii 

Japan,  2 

Java,  54 

Jemsah,  48 

Jungles,  surveying  in,  165,  168,  169 

KALA  DERIBID,  Persia,  42,  91 
Kaolin,  181 

Karroo,  South  Africa,  3  • 
Kasr-i-Cherin,  Persia,  72 
Khatan,  34,  43,  49 
Kimeridge  Clay,  17,  18,  22 
Kirta,  Baluchistan,  49 

LA  BBEA  OIL-FIELDS,  18;  oilsand, 
51-53,  94,  97,  98,  114;  pitch- 
lands,  98 

La  Lune,  Trinidad,  103,  104 

Lagon  Bouff ,  107 

Lagoons,  as  illustrating  accumulating 
vegetable  matter,  13,  14 

Lamellibranchs,  14,  62,  128;  with 
"  death-mark,  "8 

Lateral  variation,  58-66 ;  evidence  of, 
61,  62;  in  deltas,  60,  61;  in 
Pegu  Series,  125 ;  importance  of, 
65 

Lenticularity,  141,  142 

Lignite,  beds,  11;  their  formation, 
14,  15;  connection  with  petro- 
liferous beds,  16-19  ;  Cunapo 
field,  61 

Limestones,  occurrence  of  oil  in,  5, 
47 ;  advantage  over  sandstones, 
55 ;  as  affecting  quality  of  oil, 
48  ;  at  Maidan-i-Naphtun,  48 ; 
Marine,  118;  Trenton,  47 ;  Asmari, 
48,  84 ;  Spindle  Top,  48 

L'Islet  Point,  107 

Lithological  correlations  unsatis- 
factory, 65,  122 

Littoral  deposits,  evidence  from,  8 ; 
formation  of,  13-15 

Lizard  Eiver  and  Spring,  91,  92 

Location  of  wells,  132-149 ;  on  asym- 
metrical anticline,  135-139 ;  dis- 
tance apart,  149 ;  in  faulted  areas, 
145,  146 ;  on  a  monocline  or 
terrace  structure,  142,  143;  on 
symmetrical  anticline  or  dome, 
134,  140 ;  to  determine  extent  of 
field,  140,  148 

Louis  and  Gordon,  Messrs.,  94,  98 

Louisiana  oilfields,  5,  39 

Lunn,  R.,  viii 

Luristan,  Persia,  33 

MACRORIE,  B.  F.  N.,  146 
Mague  District,  Burma,  78,  83 


Maidan-i-Naphtun  oilfield,  34,  47, 
48;  sharp  folds  in,  73;  sulphur 
at,  49,  108 ;  surface  indications 
at,  90,  108 ;  unconformity  in,  84 

Maikop,  74 

Mangrove  swamps,  13 

Manjak,  45 ;  veins  of,  89,  110-114 

Map-cases,  152 

Map-making,  in  the  field,  152,  155, 
158-170  ;  traverses,  162,  168  ; 
indoor,  176 ;  sections,  178 

Maps,  use  of,  57,  151,  187;  import- 
ance of  geological,  87,  116,  144, 
151 

Marbella  Mine,  113 

Marcasite,  49 

Marmatain,  Persia,  47 ;  sulphur  at, 
49,  108 

Martin,  Dr.,  128 

Mexico,  2,  74 ;  asphalt  deposits,  102 

Migration  of  oil,  38-46 ;  as  affecting 
well-sites,  134-139 

Millstone  grit,  65 

Minbu,  Burma,  107 

Mineralization,  state  of,  122 

Miocene  strata,  14,  69 

Mollusca,  8 

Monoclines,  74 ;  locating  well  on,  142 

Morne  L'Enfer,  Trinidad,  103 

Mud- volcanoes,  of  solfataric  type,  2, 
103 ;  due  to  discharge  of  gas,  2,  41, 
103  ;  analysis  of  water  from,  104 ; 
as  a  surface  indication,  89,  103 ; 
associated  with  salt,  32 ;  at  Pitch 
Lake,  99;  in  Burma  and  Trini- 
dad, 105-107  ;  size  of,  105 

NESS,  J.,  viii 

NhangeUite,  22 

Noetling,  Dr.,  126,  127,  129 

OHIO,  72,  85,  108 

Oilfields,  near  volcanic  lines,  2;  in 
limestones,  5  ;  of  Baku,  14,  40, 
51,  52 ;  of  Louisiana,  5,  39  ;  of 
California,  5,  39,  40,  50;  of 
Eastern  United  States,  38,  40, 
71,  72;  of  Texas,  5,  33,  vide 
Spindle  Top  ;  Rio  Blanco  group, 
17,  52  ;  La  Brea  group,  18,  51-53 ; 
associated  with  lignitic  strata, 
16-19,  with  coal  seams,  20,  with 
salt  and  brine,  32 ;  of  Persia, 
vide  Luristan,Maidan-i-Naphtun; 
of  Baluchistan,  vide  Khatan  and 
Baluchistan,  vide  Burma,  Trini- 
dad, Maikop 

Oilsands,  50-53 

Oilshales,  ammonia  in,  22,  27; 
Scotch,  3,  22 

Oilshows,  vide  Shows 

Omnimeter,  154 

Organic  origin,  theories  of,  3-24 

0 


i94 


INDEX 


Origin  of  petroleum,  theories  of,  1 ; 
from  terrestrial  vegetable  matter, 
11-22;  from  sea-weeds,  22-24; 
inorganic,  from  hypogene  causes, 
2,  by  volcanic  action,  2  ;  organic, 
from  animal  matter,  4-10,  fish, 
10 ;  fossiliferous  strata,  as  bearing 
on,  8 ;  artificial,  from  peat,  27 

Orinoco,  13,  61 

Oropuche,  Trinidad,  17,  102, 107 

Oyster  beds,  62 

Ozokerite,  45  ;  as  a  surface  indication, 
110,  114 


PAKOKKU  DISTEICT,  Burma,  83 

"  Palceontographica  Indica,"  126,  127 

Palaeontology,  125,  131,  181,  vide 
Fauna 

Pala  Seco,  Trinidad,  75 

Paraffin  oils  as  contrasted  with 
asphaltic  oils,  36,  53,  91,  115, 149 
— ,  solid,  percentage  of,  36,  91 

Pascoe,  E.  H.,  72, 135 

Pauk,  16 

Peat,  processes  for  utilizing,  27 ;  as 
illustrating  formation  of  petro- 
leum, 32 

Pegu  Series  of  Burma,  8,  31,  41,  78, 
121,  124:  geological  history  of, 
63 ;  earth-movement  in,  69  ; 
dome  structure,  80 ;  uncon- 
formity with  Irrawaddy  Series, 
81,  83,  125 ;  stratigraphy  of,  126- 
128 

Pencils,  coloured,  155,  159 

Pennsylvania,  40,  72 

Persian  oilfields,  33,  34,  59,  122, 160 ; 
clay  conglomerates,  62  ;  flexures, 
68 

Peru,  74 

Petrography,  181 

Petrolene,  112 

Petroleum,  asphaltic  and  paraffin 
contrasted,  36,  53,  74,  91,  108; 
from  lime  and  sandstones  con- 
trasted, 48,  49;  its  origin,  vide 
Origin ;  percentage  of  paraffin, 
16,  36 ;  process  of  formation, 
25-36 ;  quality  of,  as  determined 
by  pressure,  28,  as  affecting 
migration,  71 

Petroliferous  phases  passing  into 
carbonaceous  and  lignitic,  15-19, 
31 

Phosphates,  difficulty  from,  in  animal- 
origin  theory,  9,  10 

Piparo,  Trinidad,  41,  103 

Pitch-lands,  at  La  Brea,  98 

Pitch  Lake  of  Trinidad,  51,  53,  114 ; 
its  formation,  93-100 

Plane-tables,  153, 169 

Point  Ligoure,  Trinidad,  section  at, 


20,   30 ;    depth    temperature   at, 

32  ;  oil  in  sea,  93 
Poole  District,  Trinidad,  114 
Porcellanites  of  Trinidad,  17-19,  29  ; 

La  Brea,  94 
Porosity    of     oilrocks,    as     affecting 

migration,  39,  71 ;    as  affecting 

sand  brought  up,  52 ;  as  affecting 

storage,  46-48 
Port  of  Spain  Harbour,  7 
Portuguese  South  Africa,  22 
Pressure,  as   condition  of  formation 

of    petroleum,   25,   28;    amount 

required,   30;    gas  pressure,   38, 

40,  134  ;  hydrostatic,  28,  38 
Princestown,  106 
Prome,  series,  121 ;  district,  126 
Prospecting,   56-59 ;  hints   for,   151- 

175 

Protractor,  154 
Pyrites,  49 

RAMRI  ISLAND,  42 

Range-tables  of  fauna,  128-131 

Redwood,  Sir  Boverton,  v 

Report  writing,    182 ;    on  a   proved 

field,  187 

Reservoir  rocks,  47-51 
Reynolds,  G.  B.,viii,  72 
Richardson,  Prof.  Clifford,  29,  41,  51, 

95-97 

Rio  Blanco,  17,  93  ;  oilsands,  52,  97 
Rogers,  C.  S.,  viii 
Russia,  20,  74 

SABE  FIELD,  Burma,  85 

Sakhalin,  2 

Salt,  associated  with  petroleum,  32, 

35,  115 
San     Fernando,    Trinidad,    19,    43; 

manjak,     111,    113;     Vistabella 

vein,  112 

Sand  grains,  contour  of,  51,  97 
Sandstones,    oil-bearing,   47,  50;   as 

affecting  quality  of  oil,  48,  49 ; 

porosity  of,  52-55 
Sangre    Grande,    Trinidad,    14,    18, 

106 

Sargasso  Sea,  23 
Scotch  oil-shales,  3,  22,  25 
Sealing  up  of  strata  by  impervious 

cover,  19,  25,  28-30 
Seaweed  origin,  theory  of,  22-24 
Sections  in  map-making,  178 
Sedimentary  beds,  formation  of,  13 
Seepages  of  oil,  as  a  surface  indica- 
tion, 90-93 
Selenite,  59,  118 
"  Shows,"  40,  46,  88,  116,  117 
Sind,  69 

Singu  oilfields,  16,  72 
Siparia,  18 
Sitshayan  shales,  121 


INDEX 


195 


Sludge,  examination  of,  6 

Sp.  gravity  of  oil,  39,  92  ;  as  affecting 
migrat  on,  71 ;  as  dependent  on 
depth,  29 

Spindle  Top,  149  ;  dome  structure, 
71;  limestone,  48 ;  sulphur  at, 
49 

Spintangi,  Baluchistan,  49 

Stratigraphy,  its  importance,  121 ; 
evidence  for  it,  122-131 

Strike,  change  of,  75,  142 

Strike-lines,  determination  of,  57,  58, 
68 

Structure,  geological,  or  secondary 
importance,  67 

Structures,  favourable  to  concentra- 
tion of  petroleum,  70- 76;  location 
of  well  on,  134-140 ;  vide  Dome, 
Terrace,  Anticline,  Monocline 

Subterranean  storage,  46-51 

Sulphur  and  its  compounds  in  petro- 
leum, 24  ;  as  a  surface  indication, 
109 ;  in  limestones,  49,  110 ;  in 
sands,  42,  49 

Sulphuretted  hydrogen,  108 

Surface  indications,  88-115 

Surveying  in  open  ground,  157 ;  in 
.  jungles,  164-169,  172,  174 ;  topo- 
graphical, 158 

Symmetrical  anticlines,  71, 134, 140 

Synclines,  73 

TACHYOMETEE,  154 

"  Tar  sands  "  of  Barbados,  109,  113 

Temperature,  vide  Depth-tempera- 
ture 

Terrace  structure,  72,  142 

Tertiary  strata  in  Burma  and  Trini- 
dad, 7,  13,  39,  41,  43,  48,  58,  61, 
64,  86,  164,  172,  189 ;  and  earth- 
movement,  68 ;  and  surveying, 
169,  171;  and  mineralization, 
124;  their  formation,  14,  17, 
19 

Texas  oilfields,  5,  33,  vide  Spindle 
Top 

"  The  Modern  Asphalt  Pavement,"  51, 
96,97 

Thompson,  A.  Beeby,  51,  52 

Tobago,  93 

"  Torpedoing,"  a  well,  53 

Traverses,  162,  168 

Trenton  limestone,  47 

Trinidad,  6,  44,  74,  75 ;  as  evidence 


for  accumulating  vegetable 
matter,  13 ;  asphalt  deposits, 
102 ;  filtered  oil,  91 ;  lignite  dis- 
trict, 18,  61;  paraffin  oils,  36, 
110;  sands  and  sandstone,  51, 
54,  109;  seepage  of  oil,  90;  sur- 
veying in,  169,  170;  Tertiary 
Series,  7,  14-17,  39,  41,  61,  64  ; 
vide  La  Brea,  Manjak,  Mud- 
volcanoes,  Pitch  Lake,  Point 
Ligoure,  Porcellanites 

Trinity  Hill  Forest  Eeserve,  90,  107 

Twingon,  oilfield,  149 

UINTAITE,  110 

Unconformabilities,  69,  81-86  i  at 
Maidan-i-Naphtun,  84 ;  in  Ohio, 
85 ;  in  Barbados,  86 ;  in  Pegu 
and  Irrawaddy  Series,  81-83 

"  Underclay,"  14,  123 

VANCE  KIVEB,  93 

Vegetable  origin  of  petroleum,  11- 
22 

Veins  of  manjak  and  ozokerite,  110 

Venezuelan  pitch  lakes,  101 

Vertical  sections,  180 

Vessiny  Elver,  94 

Vistabella  vein,  112,  113 

Volcanic  action,  as  origin  of  petro- 
leum, 2,  3 

WALL  and  Sawkins,  Messrs.,  17,  43 

Water,  necessary  in  formation  of 
petroleum,  27 ;  in  limited 
quantity,  35 ;  in  synclines,  73 

Water-sands,  50 

Well-sites,  133-150,  vide  Location  of 
Wells 

West  Indies,  2,  vide  Barbados 

West  Virginia,  20 

Winda,  Mr.,  5 

Wolgan  Valley,  Australia,  21 

"  Wrench-faults,"  77 

YAW  Valley,  Burma,  16,  31 ;  depth- 
temperature  at,  32 ;  sandstone  of, 
16,  82, 124 

Yedwet  inlier,  78,  80 

Yenangyoung  oilfields,   16,  33,  146  ; 

Yenankyat  oilfields,  16,  72,  85 

ZONES  of  fauna,  126, 127 


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